diff --git a/core/java/android/app/timezonedetector/TimeZoneDetector.java b/core/java/android/app/timezonedetector/TimeZoneDetector.java
index a71cffe5afc0b..ceab02f510629 100644
--- a/core/java/android/app/timezonedetector/TimeZoneDetector.java
+++ b/core/java/android/app/timezonedetector/TimeZoneDetector.java
@@ -95,6 +95,13 @@ public interface TimeZoneDetector {
      */
     String SHELL_COMMAND_SUGGEST_TELEPHONY_TIME_ZONE = "suggest_telephony_time_zone";
 
+    /**
+     * A shell command that enables telephony time zone fallback. See {@link
+     * com.android.server.timezonedetector.TimeZoneDetectorStrategy} for details.
+     * @hide
+     */
+    String SHELL_COMMAND_ENABLE_TELEPHONY_FALLBACK = "enable_telephony_fallback";
+
     /**
      * A shared utility method to create a {@link ManualTimeZoneSuggestion}.
      *
diff --git a/core/res/res/values/config.xml b/core/res/res/values/config.xml
index a36785f37d3f9..7262ae4157abd 100644
--- a/core/res/res/values/config.xml
+++ b/core/res/res/values/config.xml
@@ -1767,6 +1767,13 @@
          provider services. -->
     <string name="config_secondaryLocationTimeZoneProviderPackageName" translatable="false"></string>
 
+    <!-- Whether the time zone detection logic supports fall back from geolocation suggestions to
+         telephony suggestions temporarily in certain circumstances. Reduces time zone detection
+         latency during some scenarios like air travel. Only useful when both geolocation and
+         telephony time zone detection are supported on a device.
+         See com.android.server.timezonedetector.TimeZoneDetectorStrategy for more information. -->
+    <bool name="config_supportTelephonyTimeZoneFallback" translatable="false">false</bool>
+
     <!-- Whether to enable network location overlay which allows network location provider to be
          replaced by an app at run-time. When disabled, only the
          config_networkLocationProviderPackageName package will be searched for network location
diff --git a/core/res/res/values/symbols.xml b/core/res/res/values/symbols.xml
index faa99027d9ad6..5fdbff3275027 100644
--- a/core/res/res/values/symbols.xml
+++ b/core/res/res/values/symbols.xml
@@ -2237,6 +2237,7 @@
   <java-symbol type="string" name="config_primaryLocationTimeZoneProviderPackageName" />
   <java-symbol type="bool" name="config_enableSecondaryLocationTimeZoneProvider" />
   <java-symbol type="string" name="config_secondaryLocationTimeZoneProviderPackageName" />
+  <java-symbol type="bool" name="config_supportTelephonyTimeZoneFallback" />
   <java-symbol type="bool" name="config_autoResetAirplaneMode" />
   <java-symbol type="string" name="config_notificationAccessConfirmationActivity" />
   <java-symbol type="bool" name="config_killableInputMethods" />
diff --git a/services/core/java/com/android/server/timedetector/ServerFlags.java b/services/core/java/com/android/server/timedetector/ServerFlags.java
index 477ebf6402528..d24a3df56420d 100644
--- a/services/core/java/com/android/server/timedetector/ServerFlags.java
+++ b/services/core/java/com/android/server/timedetector/ServerFlags.java
@@ -65,6 +65,7 @@ public final class ServerFlags {
             KEY_LOCATION_TIME_ZONE_DETECTION_SETTING_ENABLED_DEFAULT,
             KEY_TIME_DETECTOR_LOWER_BOUND_MILLIS_OVERRIDE,
             KEY_TIME_DETECTOR_ORIGIN_PRIORITIES_OVERRIDE,
+            KEY_TIME_ZONE_DETECTOR_TELEPHONY_FALLBACK_SUPPORTED,
     })
     @Target({ ElementType.TYPE_USE, ElementType.TYPE_PARAMETER })
     @Retention(RetentionPolicy.SOURCE)
@@ -138,6 +139,14 @@ public final class ServerFlags {
             KEY_LOCATION_TIME_ZONE_DETECTION_SETTING_ENABLED_DEFAULT =
             "location_time_zone_detection_setting_enabled_default";
 
+    /**
+     * The key to control support for time zone detection falling back to telephony detection under
+     * certain circumstances.
+     */
+    public static final @DeviceConfigKey String
+            KEY_TIME_ZONE_DETECTOR_TELEPHONY_FALLBACK_SUPPORTED =
+            "time_zone_detector_telephony_fallback_supported";
+
     /**
      * The key to override the time detector origin priorities configuration. A comma-separated list
      * of strings that will be passed to {@link TimeDetectorStrategy#stringToOrigin(String)}.
diff --git a/services/core/java/com/android/server/timezonedetector/ConfigurationInternal.java b/services/core/java/com/android/server/timezonedetector/ConfigurationInternal.java
index 1a5945e3ef81f..65f077ea60e0a 100644
--- a/services/core/java/com/android/server/timezonedetector/ConfigurationInternal.java
+++ b/services/core/java/com/android/server/timezonedetector/ConfigurationInternal.java
@@ -39,6 +39,7 @@ public final class ConfigurationInternal {
 
     private final boolean mTelephonyDetectionSupported;
     private final boolean mGeoDetectionSupported;
+    private final boolean mTelephonyFallbackSupported;
     private final boolean mAutoDetectionEnabledSetting;
     private final @UserIdInt int mUserId;
     private final boolean mUserConfigAllowed;
@@ -48,6 +49,7 @@ public final class ConfigurationInternal {
     private ConfigurationInternal(Builder builder) {
         mTelephonyDetectionSupported = builder.mTelephonyDetectionSupported;
         mGeoDetectionSupported = builder.mGeoDetectionSupported;
+        mTelephonyFallbackSupported = builder.mTelephonyFallbackSupported;
         mAutoDetectionEnabledSetting = builder.mAutoDetectionEnabledSetting;
 
         mUserId = builder.mUserId;
@@ -71,6 +73,14 @@ public final class ConfigurationInternal {
         return mGeoDetectionSupported;
     }
 
+    /**
+     * Returns true if the device supports time zone detection falling back to telephony detection
+     * under certain circumstances.
+     */
+    public boolean isTelephonyFallbackSupported() {
+        return mTelephonyFallbackSupported;
+    }
+
     /** Returns the value of the auto time zone detection enabled setting. */
     public boolean getAutoDetectionEnabledSetting() {
         return mAutoDetectionEnabledSetting;
@@ -216,6 +226,7 @@ public final class ConfigurationInternal {
                 && mUserConfigAllowed == that.mUserConfigAllowed
                 && mTelephonyDetectionSupported == that.mTelephonyDetectionSupported
                 && mGeoDetectionSupported == that.mGeoDetectionSupported
+                && mTelephonyFallbackSupported == that.mTelephonyFallbackSupported
                 && mAutoDetectionEnabledSetting == that.mAutoDetectionEnabledSetting
                 && mLocationEnabledSetting == that.mLocationEnabledSetting
                 && mGeoDetectionEnabledSetting == that.mGeoDetectionEnabledSetting;
@@ -224,8 +235,8 @@ public final class ConfigurationInternal {
     @Override
     public int hashCode() {
         return Objects.hash(mUserId, mUserConfigAllowed, mTelephonyDetectionSupported,
-                mGeoDetectionSupported, mAutoDetectionEnabledSetting, mLocationEnabledSetting,
-                mGeoDetectionEnabledSetting);
+                mGeoDetectionSupported, mTelephonyFallbackSupported, mAutoDetectionEnabledSetting,
+                mLocationEnabledSetting, mGeoDetectionEnabledSetting);
     }
 
     @Override
@@ -235,6 +246,7 @@ public final class ConfigurationInternal {
                 + ", mUserConfigAllowed=" + mUserConfigAllowed
                 + ", mTelephonyDetectionSupported=" + mTelephonyDetectionSupported
                 + ", mGeoDetectionSupported=" + mGeoDetectionSupported
+                + ", mTelephonyFallbackSupported=" + mTelephonyFallbackSupported
                 + ", mAutoDetectionEnabledSetting=" + mAutoDetectionEnabledSetting
                 + ", mLocationEnabledSetting=" + mLocationEnabledSetting
                 + ", mGeoDetectionEnabledSetting=" + mGeoDetectionEnabledSetting
@@ -251,6 +263,7 @@ public final class ConfigurationInternal {
         private boolean mUserConfigAllowed;
         private boolean mTelephonyDetectionSupported;
         private boolean mGeoDetectionSupported;
+        private boolean mTelephonyFallbackSupported;
         private boolean mAutoDetectionEnabledSetting;
         private boolean mLocationEnabledSetting;
         private boolean mGeoDetectionEnabledSetting;
@@ -269,6 +282,7 @@ public final class ConfigurationInternal {
             this.mUserId = toCopy.mUserId;
             this.mUserConfigAllowed = toCopy.mUserConfigAllowed;
             this.mTelephonyDetectionSupported = toCopy.mTelephonyDetectionSupported;
+            this.mTelephonyFallbackSupported = toCopy.mTelephonyFallbackSupported;
             this.mGeoDetectionSupported = toCopy.mGeoDetectionSupported;
             this.mAutoDetectionEnabledSetting = toCopy.mAutoDetectionEnabledSetting;
             this.mLocationEnabledSetting = toCopy.mLocationEnabledSetting;
@@ -299,6 +313,15 @@ public final class ConfigurationInternal {
             return this;
         }
 
+        /**
+         * Sets whether time zone detection supports falling back to telephony detection under
+         * certain circumstances.
+         */
+        public Builder setTelephonyFallbackSupported(boolean supported) {
+            mTelephonyFallbackSupported = supported;
+            return this;
+        }
+
         /**
          * Sets the value of the automatic time zone detection enabled setting for this device.
          */
diff --git a/services/core/java/com/android/server/timezonedetector/DeviceActivityMonitor.java b/services/core/java/com/android/server/timezonedetector/DeviceActivityMonitor.java
new file mode 100644
index 0000000000000..62092ec5b4e16
--- /dev/null
+++ b/services/core/java/com/android/server/timezonedetector/DeviceActivityMonitor.java
@@ -0,0 +1,40 @@
+/*
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.server.timezonedetector;
+
+import android.annotation.NonNull;
+
+/**
+ * The interface for the class that is responsible for detecting device activities relevant to
+ * time zone detection. This interface exists to decouple parts of the time zone detector from each
+ * other and to enable easier testing.
+ *
+ * @hide
+ */
+interface DeviceActivityMonitor extends Dumpable {
+
+    /** Adds a listener. */
+    void addListener(@NonNull Listener listener);
+
+    /**
+     * A listener for device activities. See {@link DeviceActivityMonitor#addListener(Listener)}.
+     */
+    interface Listener {
+        /** A flight has completed. */
+        void onFlightComplete();
+    }
+}
diff --git a/services/core/java/com/android/server/timezonedetector/DeviceActivityMonitorImpl.java b/services/core/java/com/android/server/timezonedetector/DeviceActivityMonitorImpl.java
new file mode 100644
index 0000000000000..8c9bd3b9d0d7b
--- /dev/null
+++ b/services/core/java/com/android/server/timezonedetector/DeviceActivityMonitorImpl.java
@@ -0,0 +1,94 @@
+/*
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.server.timezonedetector;
+
+import android.annotation.NonNull;
+import android.content.ContentResolver;
+import android.content.Context;
+import android.database.ContentObserver;
+import android.os.Handler;
+import android.provider.Settings;
+import android.util.IndentingPrintWriter;
+import android.util.Slog;
+
+import com.android.internal.annotations.GuardedBy;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
+
+/**
+ * The real implementation of {@link DeviceActivityMonitor}.
+ */
+class DeviceActivityMonitorImpl implements DeviceActivityMonitor {
+
+    private static final String LOG_TAG = TimeZoneDetectorService.TAG;
+    private static final boolean DBG = TimeZoneDetectorService.DBG;
+
+    static DeviceActivityMonitor create(@NonNull Context context, @NonNull Handler handler) {
+        return new DeviceActivityMonitorImpl(context, handler);
+    }
+
+    @GuardedBy("this")
+    @NonNull
+    private final List<Listener> mListeners = new ArrayList<>();
+
+    private DeviceActivityMonitorImpl(@NonNull Context context, @NonNull Handler handler) {
+        // The way this "detects" a flight concluding is by the user explicitly turning off airplane
+        // mode. Smarter heuristics would be nice.
+        ContentResolver contentResolver = context.getContentResolver();
+        ContentObserver airplaneModeObserver = new ContentObserver(handler) {
+            @Override
+            public void onChange(boolean unused) {
+                try {
+                    int state = Settings.Global.getInt(
+                            contentResolver, Settings.Global.AIRPLANE_MODE_ON);
+                    if (state == 0) {
+                        notifyFlightComplete();
+                    }
+                } catch (Settings.SettingNotFoundException e) {
+                    Slog.e(LOG_TAG, "Unable to read airplane mode state", e);
+                }
+            }
+        };
+        contentResolver.registerContentObserver(
+                Settings.Global.getUriFor(Settings.Global.AIRPLANE_MODE_ON),
+                true /* notifyForDescendants */,
+                airplaneModeObserver);
+    }
+
+    @Override
+    public synchronized void addListener(Listener listener) {
+        Objects.requireNonNull(listener);
+        mListeners.add(listener);
+    }
+
+    private synchronized void notifyFlightComplete() {
+        if (DBG) {
+            Slog.d(LOG_TAG, "notifyFlightComplete");
+        }
+
+        for (Listener listener : mListeners) {
+            listener.onFlightComplete();
+        }
+    }
+
+    @Override
+    public void dump(IndentingPrintWriter pw, String[] args) {
+        // No-op right now: no state to dump.
+    }
+}
diff --git a/services/core/java/com/android/server/timezonedetector/EnvironmentImpl.java b/services/core/java/com/android/server/timezonedetector/EnvironmentImpl.java
index ec620b5e2a426..0ec8826cb7eb8 100644
--- a/services/core/java/com/android/server/timezonedetector/EnvironmentImpl.java
+++ b/services/core/java/com/android/server/timezonedetector/EnvironmentImpl.java
@@ -16,11 +16,13 @@
 
 package com.android.server.timezonedetector;
 
+import android.annotation.ElapsedRealtimeLong;
 import android.annotation.NonNull;
 import android.annotation.Nullable;
 import android.app.AlarmManager;
 import android.content.Context;
 import android.os.Handler;
+import android.os.SystemClock;
 import android.os.SystemProperties;
 
 import java.util.Objects;
@@ -79,4 +81,9 @@ final class EnvironmentImpl implements TimeZoneDetectorStrategyImpl.Environment
         AlarmManager alarmManager = mContext.getSystemService(AlarmManager.class);
         alarmManager.setTimeZone(zoneId);
     }
+
+    @Override
+    public @ElapsedRealtimeLong long elapsedRealtimeMillis() {
+        return SystemClock.elapsedRealtime();
+    }
 }
diff --git a/services/core/java/com/android/server/timezonedetector/MetricsTimeZoneDetectorState.java b/services/core/java/com/android/server/timezonedetector/MetricsTimeZoneDetectorState.java
index f8ba0d21224b9..0c20586ba1d67 100644
--- a/services/core/java/com/android/server/timezonedetector/MetricsTimeZoneDetectorState.java
+++ b/services/core/java/com/android/server/timezonedetector/MetricsTimeZoneDetectorState.java
@@ -111,6 +111,11 @@ public final class MetricsTimeZoneDetectorState {
         return mConfigurationInternal.isGeoDetectionSupported();
     }
 
+    /** Returns true if the device supports telephony time zone detection fallback. */
+    public boolean isTelephonyTimeZoneFallbackSupported() {
+        return mConfigurationInternal.isTelephonyFallbackSupported();
+    }
+
     /** Returns true if user's location can be used generally. */
     public boolean getUserLocationEnabledSetting() {
         return mConfigurationInternal.getLocationEnabledSetting();
diff --git a/services/core/java/com/android/server/timezonedetector/ServiceConfigAccessorImpl.java b/services/core/java/com/android/server/timezonedetector/ServiceConfigAccessorImpl.java
index be4b6bd206bf6..b9885d237bbd7 100644
--- a/services/core/java/com/android/server/timezonedetector/ServiceConfigAccessorImpl.java
+++ b/services/core/java/com/android/server/timezonedetector/ServiceConfigAccessorImpl.java
@@ -64,6 +64,7 @@ public final class ServiceConfigAccessorImpl implements ServiceConfigAccessor {
                     ServerFlags.KEY_LOCATION_TIME_ZONE_DETECTION_FEATURE_SUPPORTED,
                     ServerFlags.KEY_LOCATION_TIME_ZONE_DETECTION_SETTING_ENABLED_DEFAULT,
                     ServerFlags.KEY_LOCATION_TIME_ZONE_DETECTION_SETTING_ENABLED_OVERRIDE,
+                    ServerFlags.KEY_TIME_ZONE_DETECTOR_TELEPHONY_FALLBACK_SUPPORTED,
             }));
 
     /**
@@ -294,6 +295,7 @@ public final class ServiceConfigAccessorImpl implements ServiceConfigAccessor {
                 .setTelephonyDetectionFeatureSupported(
                         isTelephonyTimeZoneDetectionFeatureSupported())
                 .setGeoDetectionFeatureSupported(isGeoTimeZoneDetectionFeatureSupported())
+                .setTelephonyFallbackSupported(isTelephonyFallbackSupported())
                 .setAutoDetectionEnabledSetting(getAutoDetectionEnabledSetting())
                 .setUserConfigAllowed(isUserConfigAllowed(userId))
                 .setLocationEnabledSetting(getLocationEnabledSetting(userId))
@@ -549,6 +551,13 @@ public final class ServiceConfigAccessorImpl implements ServiceConfigAccessor {
         mRecordProviderStateChanges = false;
     }
 
+    private boolean isTelephonyFallbackSupported() {
+        return mServerFlags.getBoolean(
+                ServerFlags.KEY_TIME_ZONE_DETECTOR_TELEPHONY_FALLBACK_SUPPORTED,
+                getConfigBoolean(
+                        com.android.internal.R.bool.config_supportTelephonyTimeZoneFallback));
+    }
+
     private boolean getConfigBoolean(int providerEnabledConfigId) {
         Resources resources = mContext.getResources();
         return resources.getBoolean(providerEnabledConfigId);
diff --git a/services/core/java/com/android/server/timezonedetector/TimeZoneDetectorService.java b/services/core/java/com/android/server/timezonedetector/TimeZoneDetectorService.java
index e0c39ad55f470..14784cf0d5501 100644
--- a/services/core/java/com/android/server/timezonedetector/TimeZoneDetectorService.java
+++ b/services/core/java/com/android/server/timezonedetector/TimeZoneDetectorService.java
@@ -83,6 +83,16 @@ public final class TimeZoneDetectorService extends ITimeZoneDetectorService.Stub
                     ServiceConfigAccessorImpl.getInstance(context);
             TimeZoneDetectorStrategy timeZoneDetectorStrategy =
                     TimeZoneDetectorStrategyImpl.create(context, handler, serviceConfigAccessor);
+            DeviceActivityMonitor deviceActivityMonitor =
+                    DeviceActivityMonitorImpl.create(context, handler);
+
+            // Wire up the telephony fallback behavior to activity detection.
+            deviceActivityMonitor.addListener(new DeviceActivityMonitor.Listener() {
+                @Override
+                public void onFlightComplete() {
+                    timeZoneDetectorStrategy.enableTelephonyTimeZoneFallback();
+                }
+            });
 
             // Create and publish the local service for use by internal callers.
             TimeZoneDetectorInternal internal =
@@ -93,6 +103,10 @@ public final class TimeZoneDetectorService extends ITimeZoneDetectorService.Stub
             // permissioned) processes.
             TimeZoneDetectorService service = TimeZoneDetectorService.create(
                     context, handler, serviceConfigAccessor, timeZoneDetectorStrategy);
+
+            // Dump the device activity monitor when the service is dumped.
+            service.addDumpable(deviceActivityMonitor);
+
             publishBinderService(Context.TIME_ZONE_DETECTOR_SERVICE, service);
         }
     }
@@ -331,6 +345,15 @@ public final class TimeZoneDetectorService extends ITimeZoneDetectorService.Stub
         return mServiceConfigAccessor.isGeoTimeZoneDetectionFeatureSupported();
     }
 
+    /**
+     * Sends a signal to enable telephony fallback. Provided for command-line access for use
+     * during tests. This is not exposed as a binder API.
+     */
+    void enableTelephonyFallback() {
+        enforceManageTimeZoneDetectorPermission();
+        mTimeZoneDetectorStrategy.enableTelephonyTimeZoneFallback();
+    }
+
     /**
      * Registers the supplied {@link Dumpable} for dumping. When the service is dumped
      * {@link Dumpable#dump(IndentingPrintWriter, String[])} will be called on the {@code dumpable}.
diff --git a/services/core/java/com/android/server/timezonedetector/TimeZoneDetectorShellCommand.java b/services/core/java/com/android/server/timezonedetector/TimeZoneDetectorShellCommand.java
index a4a46a3c4c731..2b912ad7ba601 100644
--- a/services/core/java/com/android/server/timezonedetector/TimeZoneDetectorShellCommand.java
+++ b/services/core/java/com/android/server/timezonedetector/TimeZoneDetectorShellCommand.java
@@ -15,6 +15,7 @@
  */
 package com.android.server.timezonedetector;
 
+import static android.app.timezonedetector.TimeZoneDetector.SHELL_COMMAND_ENABLE_TELEPHONY_FALLBACK;
 import static android.app.timezonedetector.TimeZoneDetector.SHELL_COMMAND_IS_AUTO_DETECTION_ENABLED;
 import static android.app.timezonedetector.TimeZoneDetector.SHELL_COMMAND_IS_GEO_DETECTION_ENABLED;
 import static android.app.timezonedetector.TimeZoneDetector.SHELL_COMMAND_IS_GEO_DETECTION_SUPPORTED;
@@ -30,6 +31,7 @@ import static android.provider.DeviceConfig.NAMESPACE_SYSTEM_TIME;
 import static com.android.server.timedetector.ServerFlags.KEY_LOCATION_TIME_ZONE_DETECTION_FEATURE_SUPPORTED;
 import static com.android.server.timedetector.ServerFlags.KEY_LOCATION_TIME_ZONE_DETECTION_SETTING_ENABLED_DEFAULT;
 import static com.android.server.timedetector.ServerFlags.KEY_LOCATION_TIME_ZONE_DETECTION_SETTING_ENABLED_OVERRIDE;
+import static com.android.server.timedetector.ServerFlags.KEY_TIME_ZONE_DETECTOR_TELEPHONY_FALLBACK_SUPPORTED;
 
 import android.app.time.LocationTimeZoneManager;
 import android.app.time.TimeZoneConfiguration;
@@ -76,6 +78,8 @@ class TimeZoneDetectorShellCommand extends ShellCommand {
                 return runSuggestManualTimeZone();
             case SHELL_COMMAND_SUGGEST_TELEPHONY_TIME_ZONE:
                 return runSuggestTelephonyTimeZone();
+            case SHELL_COMMAND_ENABLE_TELEPHONY_FALLBACK:
+                return runEnableTelephonyFallback();
             default: {
                 return handleDefaultCommands(cmd);
             }
@@ -169,6 +173,11 @@ class TimeZoneDetectorShellCommand extends ShellCommand {
         }
     }
 
+    private int runEnableTelephonyFallback() {
+        mInterface.enableTelephonyFallback();
+        return 1;
+    }
+
     @Override
     public void onHelp() {
         final PrintWriter pw = getOutPrintWriter();
@@ -190,6 +199,13 @@ class TimeZoneDetectorShellCommand extends ShellCommand {
                 + "\n");
         pw.printf("  %s true|false\n", SHELL_COMMAND_SET_GEO_DETECTION_ENABLED);
         pw.printf("    Sets the geolocation time zone detection enabled setting.\n");
+        pw.printf("  %s\n", SHELL_COMMAND_ENABLE_TELEPHONY_FALLBACK);
+        pw.printf("    Signals that telephony time zone detection fall back can be used if"
+                + " geolocation detection is supported and enabled. This is a temporary state until"
+                + " geolocation detection becomes \"certain\". To have an effect this requires that"
+                + " the telephony fallback feature is supported on the device, see below for"
+                + " for device_config flags.\n");
+        pw.println();
         pw.printf("  %s <geolocation suggestion opts>\n",
                 SHELL_COMMAND_SUGGEST_GEO_LOCATION_TIME_ZONE);
         pw.printf("  %s <manual suggestion opts>\n",
@@ -216,6 +232,9 @@ class TimeZoneDetectorShellCommand extends ShellCommand {
         pw.printf("  %s\n", KEY_LOCATION_TIME_ZONE_DETECTION_SETTING_ENABLED_OVERRIDE);
         pw.printf("    Used to override the device's 'geolocation time zone detection enabled'"
                 + " setting [*].\n");
+        pw.printf("  %s\n", KEY_TIME_ZONE_DETECTOR_TELEPHONY_FALLBACK_SUPPORTED);
+        pw.printf("    Used to enable / disable support for telephony detection fallback. Also see"
+                + " the %s command.\n", SHELL_COMMAND_ENABLE_TELEPHONY_FALLBACK);
         pw.println();
         pw.printf("[*] To be enabled, the user must still have location = on / auto time zone"
                 + " detection = on.\n");
diff --git a/services/core/java/com/android/server/timezonedetector/TimeZoneDetectorStrategy.java b/services/core/java/com/android/server/timezonedetector/TimeZoneDetectorStrategy.java
index ede52ba84d73b..6b04adf7de618 100644
--- a/services/core/java/com/android/server/timezonedetector/TimeZoneDetectorStrategy.java
+++ b/services/core/java/com/android/server/timezonedetector/TimeZoneDetectorStrategy.java
@@ -69,6 +69,20 @@ import android.util.IndentingPrintWriter;
  * users enter areas without the necessary signals. Ultimately, with no perfect algorithm available,
  * the user is left to choose which algorithm works best for their circumstances.
  *
+ * <p>When geolocation detection is supported and enabled, in certain circumstances, such as during
+ * international travel, it makes sense to prioritize speed of detection via telephony (when
+ * available) Vs waiting for the geolocation algorithm to reach certainty. Geolocation detection can
+ * sometimes be slow to get a location fix and can require network connectivity (which cannot be
+ * assumed when users are travelling) for server-assisted location detection or time zone lookup.
+ * Therefore, as a restricted form of prioritization between geolocation and telephony algorithms,
+ * the strategy provides "telephony fallback" behavior, which can be set to "supported" via device
+ * config. Fallback mode is toggled on at runtime via {@link #enableTelephonyTimeZoneFallback()} in
+ * response to signals outside of the scope of this class. Telephony fallback allows the use of
+ * telephony suggestions to help with faster detection but only until geolocation detection
+ * provides a concrete, "certain" suggestion. After geolocation has made the first certain
+ * suggestion, telephony fallback is disabled until the next call to {@link
+ * #enableTelephonyTimeZoneFallback()}.
+ *
  * <p>Threading:
  *
  * <p>Implementations of this class must be thread-safe as calls calls like {@link
@@ -100,6 +114,13 @@ public interface TimeZoneDetectorStrategy extends Dumpable {
      */
     void suggestTelephonyTimeZone(@NonNull TelephonyTimeZoneSuggestion suggestion);
 
+    /**
+     * Tells the strategy that it can fall back to telephony detection while geolocation detection
+     * remains uncertain. {@link #suggestGeolocationTimeZone(GeolocationTimeZoneSuggestion)} can
+     * disable it again. See {@link TimeZoneDetectorStrategy} for details.
+     */
+    void enableTelephonyTimeZoneFallback();
+
     /** Generates a state snapshot for metrics. */
     @NonNull
     MetricsTimeZoneDetectorState generateMetricsState();
diff --git a/services/core/java/com/android/server/timezonedetector/TimeZoneDetectorStrategyImpl.java b/services/core/java/com/android/server/timezonedetector/TimeZoneDetectorStrategyImpl.java
index 3b6c1ead3388f..92dddacbcff5f 100644
--- a/services/core/java/com/android/server/timezonedetector/TimeZoneDetectorStrategyImpl.java
+++ b/services/core/java/com/android/server/timezonedetector/TimeZoneDetectorStrategyImpl.java
@@ -22,6 +22,7 @@ import static android.app.timezonedetector.TelephonyTimeZoneSuggestion.QUALITY_M
 import static android.app.timezonedetector.TelephonyTimeZoneSuggestion.QUALITY_MULTIPLE_ZONES_WITH_SAME_OFFSET;
 import static android.app.timezonedetector.TelephonyTimeZoneSuggestion.QUALITY_SINGLE_ZONE;
 
+import android.annotation.ElapsedRealtimeLong;
 import android.annotation.NonNull;
 import android.annotation.Nullable;
 import android.annotation.UserIdInt;
@@ -31,6 +32,7 @@ import android.app.timezonedetector.ManualTimeZoneSuggestion;
 import android.app.timezonedetector.TelephonyTimeZoneSuggestion;
 import android.content.Context;
 import android.os.Handler;
+import android.os.TimestampedValue;
 import android.util.IndentingPrintWriter;
 import android.util.LocalLog;
 import android.util.Slog;
@@ -38,6 +40,7 @@ import android.util.Slog;
 import com.android.internal.annotations.GuardedBy;
 import com.android.internal.annotations.VisibleForTesting;
 
+import java.time.Duration;
 import java.util.List;
 import java.util.Objects;
 
@@ -83,6 +86,13 @@ public final class TimeZoneDetectorStrategyImpl implements TimeZoneDetectorStrat
          * Sets the device's time zone.
          */
         void setDeviceTimeZone(@NonNull String zoneId);
+
+        /**
+         * Returns the time according to the elapsed realtime clock, the same as {@link
+         * android.os.SystemClock#elapsedRealtime()}.
+         */
+        @ElapsedRealtimeLong
+        long elapsedRealtimeMillis();
     }
 
     private static final String LOG_TAG = TimeZoneDetectorService.TAG;
@@ -190,6 +200,21 @@ public final class TimeZoneDetectorStrategyImpl implements TimeZoneDetectorStrat
     @NonNull
     private ConfigurationInternal mCurrentConfigurationInternal;
 
+    /**
+     * Whether telephony time zone detection fallback is currently enabled (when device config also
+     * allows).
+     *
+     * <p>This field is only actually used when telephony time zone fallback is supported, but the
+     * value is maintained even when it isn't supported as it can be turned on at any time via
+     * server flags. The reference time is the elapsed realtime when the mode last changed to help
+     * ordering between fallback mode switches and suggestions.
+     *
+     * <p>See {@link TimeZoneDetectorStrategy} for more information.
+     */
+    @GuardedBy("this")
+    @NonNull
+    private TimestampedValue<Boolean> mTelephonyTimeZoneFallbackEnabled;
+
     /**
      * Creates a new instance of {@link TimeZoneDetectorStrategyImpl}.
      */
@@ -205,6 +230,10 @@ public final class TimeZoneDetectorStrategyImpl implements TimeZoneDetectorStrat
     public TimeZoneDetectorStrategyImpl(@NonNull Environment environment) {
         mEnvironment = Objects.requireNonNull(environment);
 
+        // Start with telephony fallback enabled.
+        mTelephonyTimeZoneFallbackEnabled =
+                new TimestampedValue<>(mEnvironment.elapsedRealtimeMillis(), true);
+
         synchronized (this) {
             mEnvironment.setConfigurationInternalChangeListener(
                     this::handleConfigurationInternalChanged);
@@ -233,6 +262,10 @@ public final class TimeZoneDetectorStrategyImpl implements TimeZoneDetectorStrat
         // are made in a sensible order and the most recent is always the best one to use.
         mLatestGeoLocationSuggestion.set(suggestion);
 
+        // Update the mTelephonyTimeZoneFallbackEnabled state if needed: a certain suggestion
+        // will usually disable telephony fallback mode if it is currently enabled.
+        disableTelephonyFallbackIfNeeded();
+
         // Now perform auto time zone detection. The new suggestion may be used to modify the
         // time zone setting.
         String reason = "New geolocation time zone suggested. suggestion=" + suggestion;
@@ -303,6 +336,43 @@ public final class TimeZoneDetectorStrategyImpl implements TimeZoneDetectorStrat
         doAutoTimeZoneDetection(currentUserConfig, reason);
     }
 
+    @Override
+    public synchronized void enableTelephonyTimeZoneFallback() {
+        // Only do any work if fallback is currently not enabled.
+        if (!mTelephonyTimeZoneFallbackEnabled.getValue()) {
+            ConfigurationInternal currentUserConfig = mCurrentConfigurationInternal;
+            if (DBG) {
+                Slog.d(LOG_TAG, "enableTelephonyTimeZoneFallbackMode"
+                        + ": currentUserConfig=" + currentUserConfig);
+            }
+
+            final boolean fallbackEnabled = true;
+            mTelephonyTimeZoneFallbackEnabled = new TimestampedValue<>(
+                    mEnvironment.elapsedRealtimeMillis(), fallbackEnabled);
+
+            // mTelephonyTimeZoneFallbackEnabled and mLatestGeoLocationSuggestion interact.
+            // If there is currently a certain geolocation suggestion, then the telephony fallback
+            // value needs to be considered after changing it.
+            // With the way that the mTelephonyTimeZoneFallbackEnabled time is currently chosen
+            // above, and the fact that geolocation suggestions should never have a time in the
+            // future, the following call will be a no-op, and telephony fallback will remain
+            // enabled. This comment / call is left as a reminder that it is possible for there to
+            // be a current, "certain" geolocation suggestion when this signal arrives and it is
+            // intentional that fallback stays enabled in this case. The choice to do this
+            // is mostly for symmetry WRT the case where fallback is enabled and an old "certain"
+            // geolocation is received; that would also leave telephony fallback enabled.
+            // This choice means that telephony fallback will remain enabled until a new "certain"
+            // geolocation suggestion is received. If, instead, the next geolocation is "uncertain",
+            // then telephony fallback will occur.
+            disableTelephonyFallbackIfNeeded();
+
+            if (currentUserConfig.isTelephonyFallbackSupported()) {
+                String reason = "enableTelephonyTimeZoneFallbackMode";
+                doAutoTimeZoneDetection(currentUserConfig, reason);
+            }
+        }
+    }
+
     @Override
     @NonNull
     public synchronized MetricsTimeZoneDetectorState generateMetricsState() {
@@ -361,7 +431,32 @@ public final class TimeZoneDetectorStrategyImpl implements TimeZoneDetectorStrat
         // Use the correct algorithm based on the user's current configuration. If it changes, then
         // detection will be re-run.
         if (currentUserConfig.getGeoDetectionEnabledBehavior()) {
-            doGeolocationTimeZoneDetection(detectionReason);
+            boolean isGeoDetectionCertain = doGeolocationTimeZoneDetection(detectionReason);
+
+            // When geolocation detection is uncertain of the time zone, telephony detection
+            // can be used if telephony fallback is enabled and supported.
+            if (!isGeoDetectionCertain
+                    && mTelephonyTimeZoneFallbackEnabled.getValue()
+                    && currentUserConfig.isTelephonyFallbackSupported()) {
+
+                // This "only look at telephony if geolocation is uncertain" approach is
+                // deliberate to try to keep the logic simple and keep telephony and geolocation
+                // detection decoupled: when geolocation detection is in use, it is fully
+                // trusted and the most recent "certain" geolocation suggestion available will
+                // be used, even if the information it is based on is quite old.
+                // There could be newer telephony suggestions available, but telephony
+                // suggestions tend not to be withdrawn when they should be, and are based on
+                // combining information like MCC and NITZ signals, which could have been
+                // received at different times; thus it is hard to say what time the suggestion
+                // is actually "for" and reason clearly about ordering between telephony and
+                // geolocation suggestions.
+                //
+                // This approach is reliant on the location_time_zone_manager (and the location
+                // time zone providers it manages) correctly sending "uncertain" suggestions
+                // when the current location is unknown so that telephony fallback will actually be
+                // used.
+                doTelephonyTimeZoneDetection(detectionReason + ", telephony fallback mode");
+            }
         } else  {
             doTelephonyTimeZoneDetection(detectionReason);
         }
@@ -371,21 +466,29 @@ public final class TimeZoneDetectorStrategyImpl implements TimeZoneDetectorStrat
      * Detects the time zone using the latest available geolocation time zone suggestion, if one is
      * available. The outcome can be that this strategy becomes / remains un-opinionated and nothing
      * is set.
+     *
+     * @return true if geolocation time zone detection was certain of the time zone, false if it is
+     * uncertain
      */
     @GuardedBy("this")
-    private void doGeolocationTimeZoneDetection(@NonNull String detectionReason) {
+    private boolean doGeolocationTimeZoneDetection(@NonNull String detectionReason) {
         GeolocationTimeZoneSuggestion latestGeolocationSuggestion =
                 mLatestGeoLocationSuggestion.get();
         if (latestGeolocationSuggestion == null) {
-            return;
+            return false;
         }
 
         List<String> zoneIds = latestGeolocationSuggestion.getZoneIds();
-        if (zoneIds == null || zoneIds.isEmpty()) {
-            // This means the client has become uncertain about the time zone or it is certain there
-            // is no known zone. In either case we must leave the existing time zone setting as it
-            // is.
-            return;
+        if (zoneIds == null) {
+            // This means the originator of the suggestion is uncertain about the time zone. The
+            // existing time zone setting must be left as it is but detection can go on looking for
+            // a different answer elsewhere.
+            return false;
+        } else if (zoneIds.isEmpty()) {
+            // This means the originator is certain there is no time zone. The existing time zone
+            // setting must be left as it is and detection must not go looking for a different
+            // answer elsewhere.
+            return true;
         }
 
         // GeolocationTimeZoneSuggestion has no measure of quality. We assume all suggestions are
@@ -404,6 +507,34 @@ public final class TimeZoneDetectorStrategyImpl implements TimeZoneDetectorStrat
             zoneId = zoneIds.get(0);
         }
         setDeviceTimeZoneIfRequired(zoneId, detectionReason);
+        return true;
+    }
+
+    /**
+     * Sets the mTelephonyTimeZoneFallbackEnabled state to {@code false} if the latest geo
+     * suggestion is a "certain" suggestion that comes after the time when telephony fallback was
+     * enabled.
+     */
+    @GuardedBy("this")
+    private void disableTelephonyFallbackIfNeeded() {
+        GeolocationTimeZoneSuggestion suggestion = mLatestGeoLocationSuggestion.get();
+        boolean isLatestSuggestionCertain = suggestion != null && suggestion.getZoneIds() != null;
+        if (isLatestSuggestionCertain && mTelephonyTimeZoneFallbackEnabled.getValue()) {
+            // This transition ONLY changes mTelephonyTimeZoneFallbackEnabled from
+            // true -> false. See mTelephonyTimeZoneFallbackEnabled javadocs for details.
+
+            // Telephony fallback will be disabled after a "certain" suggestion is processed
+            // if and only if the location information it is based on is from after telephony
+            // fallback was enabled.
+            boolean latestSuggestionIsNewerThanFallbackEnabled =
+                    suggestion.getEffectiveFromElapsedMillis()
+                            > mTelephonyTimeZoneFallbackEnabled.getReferenceTimeMillis();
+            if (latestSuggestionIsNewerThanFallbackEnabled) {
+                final boolean fallbackEnabled = false;
+                mTelephonyTimeZoneFallbackEnabled = new TimestampedValue<>(
+                        mEnvironment.elapsedRealtimeMillis(), fallbackEnabled);
+            }
+        }
     }
 
     /**
@@ -556,6 +687,12 @@ public final class TimeZoneDetectorStrategyImpl implements TimeZoneDetectorStrat
                 + mEnvironment.isDeviceTimeZoneInitialized());
         ipw.println("mEnvironment.getDeviceTimeZone()=" + mEnvironment.getDeviceTimeZone());
 
+        ipw.println("Misc state:");
+        ipw.increaseIndent(); // level 2
+        ipw.println("mTelephonyTimeZoneFallbackEnabled="
+                + formatDebugString(mTelephonyTimeZoneFallbackEnabled));
+        ipw.decreaseIndent(); // level 2
+
         ipw.println("Time zone change log:");
         ipw.increaseIndent(); // level 2
         mTimeZoneChangesLog.dump(ipw);
@@ -603,6 +740,11 @@ public final class TimeZoneDetectorStrategyImpl implements TimeZoneDetectorStrat
         return mLatestGeoLocationSuggestion.get();
     }
 
+    @VisibleForTesting
+    public synchronized boolean isTelephonyFallbackEnabledForTests() {
+        return mTelephonyTimeZoneFallbackEnabled.getValue();
+    }
+
     /**
      * A {@link TelephonyTimeZoneSuggestion} with additional qualifying metadata.
      */
@@ -652,4 +794,8 @@ public final class TimeZoneDetectorStrategyImpl implements TimeZoneDetectorStrat
                     + '}';
         }
     }
+
+    private static String formatDebugString(TimestampedValue<?> value) {
+        return value.getValue() + " @ " + Duration.ofMillis(value.getReferenceTimeMillis());
+    }
 }
diff --git a/services/tests/servicestests/src/com/android/server/timezonedetector/ConfigurationInternalTest.java b/services/tests/servicestests/src/com/android/server/timezonedetector/ConfigurationInternalTest.java
index 809b2f9f977af..6ee6020c79852 100644
--- a/services/tests/servicestests/src/com/android/server/timezonedetector/ConfigurationInternalTest.java
+++ b/services/tests/servicestests/src/com/android/server/timezonedetector/ConfigurationInternalTest.java
@@ -48,6 +48,7 @@ public class ConfigurationInternalTest {
                 .setUserConfigAllowed(true)
                 .setTelephonyDetectionFeatureSupported(true)
                 .setGeoDetectionFeatureSupported(true)
+                .setTelephonyFallbackSupported(false)
                 .setAutoDetectionEnabledSetting(true)
                 .setLocationEnabledSetting(true)
                 .setGeoDetectionEnabledSetting(true)
@@ -110,6 +111,7 @@ public class ConfigurationInternalTest {
                 .setUserConfigAllowed(false)
                 .setTelephonyDetectionFeatureSupported(true)
                 .setGeoDetectionFeatureSupported(true)
+                .setTelephonyFallbackSupported(false)
                 .setAutoDetectionEnabledSetting(true)
                 .setLocationEnabledSetting(true)
                 .setGeoDetectionEnabledSetting(true)
@@ -174,6 +176,7 @@ public class ConfigurationInternalTest {
                 .setUserConfigAllowed(true)
                 .setTelephonyDetectionFeatureSupported(false)
                 .setGeoDetectionFeatureSupported(false)
+                .setTelephonyFallbackSupported(false)
                 .setAutoDetectionEnabledSetting(true)
                 .setLocationEnabledSetting(true)
                 .setGeoDetectionEnabledSetting(true)
@@ -236,6 +239,7 @@ public class ConfigurationInternalTest {
                 .setUserConfigAllowed(true)
                 .setTelephonyDetectionFeatureSupported(true)
                 .setGeoDetectionFeatureSupported(false)
+                .setTelephonyFallbackSupported(false)
                 .setAutoDetectionEnabledSetting(true)
                 .setLocationEnabledSetting(true)
                 .setGeoDetectionEnabledSetting(true)
@@ -288,4 +292,18 @@ public class ConfigurationInternalTest {
             assertTrue(configuration.isGeoDetectionEnabled());
         }
     }
+
+    @Test
+    public void test_telephonyFallbackSupported() {
+        ConfigurationInternal config = new ConfigurationInternal.Builder(ARBITRARY_USER_ID)
+                .setUserConfigAllowed(true)
+                .setTelephonyDetectionFeatureSupported(true)
+                .setGeoDetectionFeatureSupported(false)
+                .setTelephonyFallbackSupported(true)
+                .setAutoDetectionEnabledSetting(true)
+                .setLocationEnabledSetting(true)
+                .setGeoDetectionEnabledSetting(true)
+                .build();
+        assertTrue(config.isTelephonyFallbackSupported());
+    }
 }
diff --git a/services/tests/servicestests/src/com/android/server/timezonedetector/FakeTimeZoneDetectorStrategy.java b/services/tests/servicestests/src/com/android/server/timezonedetector/FakeTimeZoneDetectorStrategy.java
index ee3195e92c6da..2d0dca25465b9 100644
--- a/services/tests/servicestests/src/com/android/server/timezonedetector/FakeTimeZoneDetectorStrategy.java
+++ b/services/tests/servicestests/src/com/android/server/timezonedetector/FakeTimeZoneDetectorStrategy.java
@@ -50,6 +50,11 @@ class FakeTimeZoneDetectorStrategy implements TimeZoneDetectorStrategy {
         mLastTelephonySuggestion = timeZoneSuggestion;
     }
 
+    @Override
+    public void enableTelephonyTimeZoneFallback() {
+        throw new UnsupportedOperationException();
+    }
+
     @Override
     public MetricsTimeZoneDetectorState generateMetricsState() {
         throw new UnsupportedOperationException();
diff --git a/services/tests/servicestests/src/com/android/server/timezonedetector/TimeZoneDetectorServiceTest.java b/services/tests/servicestests/src/com/android/server/timezonedetector/TimeZoneDetectorServiceTest.java
index ce3b78e2b5f78..13317747c95cc 100644
--- a/services/tests/servicestests/src/com/android/server/timezonedetector/TimeZoneDetectorServiceTest.java
+++ b/services/tests/servicestests/src/com/android/server/timezonedetector/TimeZoneDetectorServiceTest.java
@@ -375,6 +375,7 @@ public class TimeZoneDetectorServiceTest {
         return new ConfigurationInternal.Builder(ARBITRARY_USER_ID)
                 .setTelephonyDetectionFeatureSupported(true)
                 .setGeoDetectionFeatureSupported(true)
+                .setTelephonyFallbackSupported(false)
                 .setUserConfigAllowed(true)
                 .setAutoDetectionEnabledSetting(autoDetectionEnabled)
                 .setLocationEnabledSetting(geoDetectionEnabled)
diff --git a/services/tests/servicestests/src/com/android/server/timezonedetector/TimeZoneDetectorStrategyImplTest.java b/services/tests/servicestests/src/com/android/server/timezonedetector/TimeZoneDetectorStrategyImplTest.java
index 91e8d165f4bea..ef1b4f58dd769 100644
--- a/services/tests/servicestests/src/com/android/server/timezonedetector/TimeZoneDetectorStrategyImplTest.java
+++ b/services/tests/servicestests/src/com/android/server/timezonedetector/TimeZoneDetectorStrategyImplTest.java
@@ -60,6 +60,7 @@ import java.util.function.Function;
 public class TimeZoneDetectorStrategyImplTest {
 
     private static final @UserIdInt int USER_ID = 9876;
+    private static final long ARBITRARY_ELAPSED_REALTIME_MILLIS = 1234;
     /** A time zone used for initialization that does not occur elsewhere in tests. */
     private static final String ARBITRARY_TIME_ZONE_ID = "Etc/UTC";
     private static final int SLOT_INDEX1 = 10000;
@@ -89,6 +90,7 @@ public class TimeZoneDetectorStrategyImplTest {
                     .setUserConfigAllowed(false)
                     .setTelephonyDetectionFeatureSupported(true)
                     .setGeoDetectionFeatureSupported(true)
+                    .setTelephonyFallbackSupported(false)
                     .setAutoDetectionEnabledSetting(false)
                     .setLocationEnabledSetting(true)
                     .setGeoDetectionEnabledSetting(false)
@@ -99,6 +101,7 @@ public class TimeZoneDetectorStrategyImplTest {
                     .setUserConfigAllowed(false)
                     .setTelephonyDetectionFeatureSupported(true)
                     .setGeoDetectionFeatureSupported(true)
+                    .setTelephonyFallbackSupported(false)
                     .setAutoDetectionEnabledSetting(true)
                     .setLocationEnabledSetting(true)
                     .setGeoDetectionEnabledSetting(true)
@@ -109,6 +112,7 @@ public class TimeZoneDetectorStrategyImplTest {
                     .setUserConfigAllowed(true)
                     .setTelephonyDetectionFeatureSupported(false)
                     .setGeoDetectionFeatureSupported(false)
+                    .setTelephonyFallbackSupported(false)
                     .setAutoDetectionEnabledSetting(false)
                     .setLocationEnabledSetting(true)
                     .setGeoDetectionEnabledSetting(false)
@@ -119,6 +123,7 @@ public class TimeZoneDetectorStrategyImplTest {
                     .setUserConfigAllowed(true)
                     .setTelephonyDetectionFeatureSupported(true)
                     .setGeoDetectionFeatureSupported(true)
+                    .setTelephonyFallbackSupported(false)
                     .setAutoDetectionEnabledSetting(false)
                     .setLocationEnabledSetting(true)
                     .setGeoDetectionEnabledSetting(false)
@@ -128,6 +133,7 @@ public class TimeZoneDetectorStrategyImplTest {
             new ConfigurationInternal.Builder(USER_ID)
                     .setTelephonyDetectionFeatureSupported(true)
                     .setGeoDetectionFeatureSupported(true)
+                    .setTelephonyFallbackSupported(false)
                     .setUserConfigAllowed(true)
                     .setAutoDetectionEnabledSetting(true)
                     .setLocationEnabledSetting(true)
@@ -138,6 +144,7 @@ public class TimeZoneDetectorStrategyImplTest {
             new ConfigurationInternal.Builder(USER_ID)
                     .setTelephonyDetectionFeatureSupported(true)
                     .setGeoDetectionFeatureSupported(true)
+                    .setTelephonyFallbackSupported(false)
                     .setUserConfigAllowed(true)
                     .setAutoDetectionEnabledSetting(true)
                     .setLocationEnabledSetting(true)
@@ -147,9 +154,6 @@ public class TimeZoneDetectorStrategyImplTest {
     private TimeZoneDetectorStrategyImpl mTimeZoneDetectorStrategy;
     private FakeEnvironment mFakeEnvironment;
 
-    // A fake source of time for suggestions. This will typically be incremented after every use.
-    @ElapsedRealtimeLong private long mElapsedRealtimeMillis;
-
     @Before
     public void setUp() {
         mFakeEnvironment = new FakeEnvironment();
@@ -752,6 +756,204 @@ public class TimeZoneDetectorStrategyImplTest {
                 mTimeZoneDetectorStrategy.getLatestGeolocationSuggestion());
     }
 
+    @Test
+    public void testTelephonyFallback() {
+        ConfigurationInternal config = new ConfigurationInternal.Builder(
+                CONFIG_AUTO_ENABLED_GEO_ENABLED)
+                .setTelephonyFallbackSupported(true)
+                .build();
+
+        Script script = new Script()
+                .initializeClock(ARBITRARY_ELAPSED_REALTIME_MILLIS)
+                .initializeTimeZoneSetting(ARBITRARY_TIME_ZONE_ID)
+                .simulateConfigurationInternalChange(config)
+                .resetConfigurationTracking();
+
+        // Confirm initial state is as expected.
+        script.verifyTelephonyFallbackIsEnabled(true)
+                .verifyTimeZoneNotChanged();
+
+        // Although geolocation detection is enabled, telephony fallback should be used initially
+        // and until a suitable "certain" geolocation suggestion is received.
+        {
+            TelephonyTimeZoneSuggestion telephonySuggestion = createTelephonySuggestion(
+                    SLOT_INDEX1, MATCH_TYPE_NETWORK_COUNTRY_AND_OFFSET, QUALITY_SINGLE_ZONE,
+                    "Europe/Paris");
+            script.simulateIncrementClock()
+                    .simulateTelephonyTimeZoneSuggestion(telephonySuggestion)
+                    .verifyTimeZoneChangedAndReset(telephonySuggestion)
+                    .verifyTelephonyFallbackIsEnabled(true);
+        }
+
+        // Receiving an "uncertain" geolocation suggestion should have no effect.
+        {
+            GeolocationTimeZoneSuggestion uncertainGeolocationSuggestion =
+                    createUncertainGeolocationSuggestion();
+            script.simulateIncrementClock()
+                    .simulateGeolocationTimeZoneSuggestion(uncertainGeolocationSuggestion)
+                    .verifyTimeZoneNotChanged()
+                    .verifyTelephonyFallbackIsEnabled(true);
+        }
+
+        // Receiving a "certain" geolocation suggestion should disable telephony fallback mode.
+        {
+            GeolocationTimeZoneSuggestion geolocationSuggestion =
+                    createCertainGeolocationSuggestion("Europe/London");
+            script.simulateIncrementClock()
+                    .simulateGeolocationTimeZoneSuggestion(geolocationSuggestion)
+                    .verifyTimeZoneChangedAndReset(geolocationSuggestion)
+                    .verifyTelephonyFallbackIsEnabled(false);
+        }
+
+        // Used to record the last telephony suggestion received, which will be used when fallback
+        // takes place.
+        TelephonyTimeZoneSuggestion lastTelephonySuggestion;
+
+        // Telephony suggestions should now be ignored and geolocation detection is "in control".
+        {
+            TelephonyTimeZoneSuggestion telephonySuggestion = createTelephonySuggestion(
+                    SLOT_INDEX1, MATCH_TYPE_NETWORK_COUNTRY_AND_OFFSET, QUALITY_SINGLE_ZONE,
+                    "Europe/Berlin");
+            script.simulateIncrementClock()
+                    .simulateTelephonyTimeZoneSuggestion(telephonySuggestion)
+                    .verifyTimeZoneNotChanged()
+                    .verifyTelephonyFallbackIsEnabled(false);
+            lastTelephonySuggestion = telephonySuggestion;
+        }
+
+        // Geolocation suggestions should continue to be used as normal (previous telephony
+        // suggestions are not used, even when the geolocation suggestion is uncertain).
+        {
+            GeolocationTimeZoneSuggestion geolocationSuggestion =
+                    createCertainGeolocationSuggestion("Europe/Rome");
+            script.simulateIncrementClock()
+                    .simulateGeolocationTimeZoneSuggestion(geolocationSuggestion)
+                    .verifyTimeZoneChangedAndReset(geolocationSuggestion)
+                    .verifyTelephonyFallbackIsEnabled(false);
+
+            GeolocationTimeZoneSuggestion uncertainGeolocationSuggestion =
+                    createUncertainGeolocationSuggestion();
+            script.simulateIncrementClock()
+                    .simulateGeolocationTimeZoneSuggestion(uncertainGeolocationSuggestion)
+                    .verifyTimeZoneNotChanged()
+                    .verifyTelephonyFallbackIsEnabled(false);
+
+            script.simulateIncrementClock()
+                    .simulateGeolocationTimeZoneSuggestion(geolocationSuggestion)
+                    // No change needed, device will already be set to Europe/Rome.
+                    .verifyTimeZoneNotChanged()
+                    .verifyTelephonyFallbackIsEnabled(false);
+        }
+
+        // Enable telephony fallback. Nothing will change, because the geolocation is still certain,
+        // but fallback will remain enabled.
+        {
+            script.simulateIncrementClock()
+                    .simulateEnableTelephonyFallback()
+                    .verifyTimeZoneNotChanged()
+                    .verifyTelephonyFallbackIsEnabled(true);
+        }
+
+        // Make the geolocation algorithm uncertain.
+        {
+            GeolocationTimeZoneSuggestion uncertainGeolocationSuggestion =
+                    createUncertainGeolocationSuggestion();
+            script.simulateIncrementClock()
+                    .simulateGeolocationTimeZoneSuggestion(uncertainGeolocationSuggestion)
+                    .verifyTimeZoneChangedAndReset(lastTelephonySuggestion)
+                    .verifyTelephonyFallbackIsEnabled(true);
+        }
+
+        // Make the geolocation algorithm certain, disabling telephony fallback.
+        {
+            GeolocationTimeZoneSuggestion geolocationSuggestion =
+                    createCertainGeolocationSuggestion("Europe/Lisbon");
+            script.simulateIncrementClock()
+                    .simulateGeolocationTimeZoneSuggestion(geolocationSuggestion)
+                    .verifyTimeZoneChangedAndReset(geolocationSuggestion)
+                    .verifyTelephonyFallbackIsEnabled(false);
+
+        }
+
+        // Demonstrate what happens when geolocation is uncertain when telephony fallback is
+        // enabled.
+        {
+            GeolocationTimeZoneSuggestion uncertainGeolocationSuggestion =
+                    createUncertainGeolocationSuggestion();
+            script.simulateIncrementClock()
+                    .simulateGeolocationTimeZoneSuggestion(uncertainGeolocationSuggestion)
+                    .verifyTimeZoneNotChanged()
+                    .verifyTelephonyFallbackIsEnabled(false)
+                    .simulateEnableTelephonyFallback()
+                    .verifyTimeZoneChangedAndReset(lastTelephonySuggestion)
+                    .verifyTelephonyFallbackIsEnabled(true);
+        }
+    }
+
+    @Test
+    public void testTelephonyFallback_noTelephonySuggestionToFallBackTo() {
+        ConfigurationInternal config = new ConfigurationInternal.Builder(
+                CONFIG_AUTO_ENABLED_GEO_ENABLED)
+                .setTelephonyFallbackSupported(true)
+                .build();
+
+        Script script = new Script()
+                .initializeClock(ARBITRARY_ELAPSED_REALTIME_MILLIS)
+                .initializeTimeZoneSetting(ARBITRARY_TIME_ZONE_ID)
+                .simulateConfigurationInternalChange(config)
+                .resetConfigurationTracking();
+
+        // Confirm initial state is as expected.
+        script.verifyTelephonyFallbackIsEnabled(true)
+                .verifyTimeZoneNotChanged();
+
+        // Receiving an "uncertain" geolocation suggestion should have no effect.
+        {
+            GeolocationTimeZoneSuggestion uncertainGeolocationSuggestion =
+                    createUncertainGeolocationSuggestion();
+            script.simulateIncrementClock()
+                    .simulateGeolocationTimeZoneSuggestion(uncertainGeolocationSuggestion)
+                    .verifyTimeZoneNotChanged()
+                    .verifyTelephonyFallbackIsEnabled(true);
+        }
+
+        // Make an uncertain geolocation suggestion, there is no telephony suggestion to fall back
+        // to
+        {
+            GeolocationTimeZoneSuggestion uncertainGeolocationSuggestion =
+                    createUncertainGeolocationSuggestion();
+            script.simulateIncrementClock()
+                    .simulateGeolocationTimeZoneSuggestion(uncertainGeolocationSuggestion)
+                    .verifyTimeZoneNotChanged()
+                    .verifyTelephonyFallbackIsEnabled(true);
+        }
+
+        // Similar to the case above, but force a fallback attempt after making a "certain"
+        // geolocation suggestion.
+        // Geolocation suggestions should continue to be used as normal (previous telephony
+        // suggestions are not used, even when the geolocation suggestion is uncertain).
+        {
+            GeolocationTimeZoneSuggestion geolocationSuggestion =
+                    createCertainGeolocationSuggestion("Europe/Rome");
+            script.simulateIncrementClock()
+                    .simulateGeolocationTimeZoneSuggestion(geolocationSuggestion)
+                    .verifyTimeZoneChangedAndReset(geolocationSuggestion)
+                    .verifyTelephonyFallbackIsEnabled(false);
+
+            GeolocationTimeZoneSuggestion uncertainGeolocationSuggestion =
+                    createUncertainGeolocationSuggestion();
+            script.simulateIncrementClock()
+                    .simulateGeolocationTimeZoneSuggestion(uncertainGeolocationSuggestion)
+                    .verifyTimeZoneNotChanged()
+                    .verifyTelephonyFallbackIsEnabled(false);
+
+            script.simulateIncrementClock()
+                    .simulateEnableTelephonyFallback()
+                    .verifyTimeZoneNotChanged()
+                    .verifyTelephonyFallbackIsEnabled(true);
+        }
+    }
+
     @Test
     public void testGenerateMetricsState() {
         ConfigurationInternal expectedInternalConfig = CONFIG_AUTO_DISABLED_GEO_DISABLED;
@@ -835,6 +1037,8 @@ public class TimeZoneDetectorStrategyImplTest {
         assertEquals(config.isTelephonyDetectionSupported(),
                 actualState.isTelephonyDetectionSupported());
         assertEquals(config.isGeoDetectionSupported(), actualState.isGeoDetectionSupported());
+        assertEquals(config.isTelephonyFallbackSupported(),
+                actualState.isTelephonyTimeZoneFallbackSupported());
         assertEquals(config.getAutoDetectionEnabledSetting(),
                 actualState.getAutoDetectionEnabledSetting());
         assertEquals(config.getGeoDetectionEnabledSetting(),
@@ -865,7 +1069,7 @@ public class TimeZoneDetectorStrategyImplTest {
 
     private GeolocationTimeZoneSuggestion createUncertainGeolocationSuggestion() {
         return GeolocationTimeZoneSuggestion.createCertainSuggestion(
-                mElapsedRealtimeMillis++, null);
+                mFakeEnvironment.elapsedRealtimeMillis(), null);
     }
 
     private GeolocationTimeZoneSuggestion createCertainGeolocationSuggestion(
@@ -874,7 +1078,7 @@ public class TimeZoneDetectorStrategyImplTest {
 
         GeolocationTimeZoneSuggestion suggestion =
                 GeolocationTimeZoneSuggestion.createCertainSuggestion(
-                        mElapsedRealtimeMillis++, Arrays.asList(zoneIds));
+                        mFakeEnvironment.elapsedRealtimeMillis(), Arrays.asList(zoneIds));
         suggestion.addDebugInfo("Test suggestion");
         return suggestion;
     }
@@ -883,16 +1087,25 @@ public class TimeZoneDetectorStrategyImplTest {
 
         private final TestState<String> mTimeZoneId = new TestState<>();
         private ConfigurationInternal mConfigurationInternal;
+        private @ElapsedRealtimeLong long mElapsedRealtimeMillis;
         private ConfigurationChangeListener mConfigurationInternalChangeListener;
 
         void initializeConfig(ConfigurationInternal configurationInternal) {
             mConfigurationInternal = configurationInternal;
         }
 
+        void initializeClock(@ElapsedRealtimeLong long elapsedRealtimeMillis) {
+            mElapsedRealtimeMillis = elapsedRealtimeMillis;
+        }
+
         void initializeTimeZoneSetting(String zoneId) {
             mTimeZoneId.init(zoneId);
         }
 
+        void incrementClock() {
+            mElapsedRealtimeMillis++;
+        }
+
         @Override
         public void setConfigurationInternalChangeListener(ConfigurationChangeListener listener) {
             mConfigurationInternalChangeListener = listener;
@@ -936,6 +1149,12 @@ public class TimeZoneDetectorStrategyImplTest {
         void commitAllChanges() {
             mTimeZoneId.commitLatest();
         }
+
+        @Override
+        @ElapsedRealtimeLong
+        public long elapsedRealtimeMillis() {
+            return mElapsedRealtimeMillis;
+        }
     }
 
     /**
@@ -949,6 +1168,16 @@ public class TimeZoneDetectorStrategyImplTest {
             return this;
         }
 
+        Script initializeClock(long elapsedRealtimeMillis) {
+            mFakeEnvironment.initializeClock(elapsedRealtimeMillis);
+            return this;
+        }
+
+        Script simulateIncrementClock() {
+            mFakeEnvironment.incrementClock();
+            return this;
+        }
+
         /**
          * Simulates the user / user's configuration changing.
          */
@@ -1008,6 +1237,15 @@ public class TimeZoneDetectorStrategyImplTest {
             return this;
         }
 
+        /**
+         * Simulates the time zone detection strategty receiving a signal that allows it to do
+         * telephony fallback.
+         */
+        Script simulateEnableTelephonyFallback() {
+            mTimeZoneDetectorStrategy.enableTelephonyTimeZoneFallback();
+            return this;
+        }
+
         /**
          * Confirms that the device's time zone has not been set by previous actions since the test
          * state was last reset.
@@ -1044,6 +1282,13 @@ public class TimeZoneDetectorStrategyImplTest {
             return this;
         }
 
+        /** Verifies the state for telephony fallback. */
+        Script verifyTelephonyFallbackIsEnabled(boolean expectedEnabled) {
+            assertEquals(expectedEnabled,
+                    mTimeZoneDetectorStrategy.isTelephonyFallbackEnabledForTests());
+            return this;
+        }
+
         Script resetConfigurationTracking() {
             mFakeEnvironment.commitAllChanges();
             return this;
diff --git a/services/tests/servicestests/src/com/android/server/timezonedetector/location/TestSupport.java b/services/tests/servicestests/src/com/android/server/timezonedetector/location/TestSupport.java
index e3da90e343ed8..a2df31305c2f7 100644
--- a/services/tests/servicestests/src/com/android/server/timezonedetector/location/TestSupport.java
+++ b/services/tests/servicestests/src/com/android/server/timezonedetector/location/TestSupport.java
@@ -46,6 +46,7 @@ final class TestSupport {
                 .setUserConfigAllowed(true)
                 .setTelephonyDetectionFeatureSupported(true)
                 .setGeoDetectionFeatureSupported(true)
+                .setTelephonyFallbackSupported(false)
                 .setAutoDetectionEnabledSetting(true)
                 .setLocationEnabledSetting(true)
                 .setGeoDetectionEnabledSetting(geoDetectionEnabledSetting)