diff --git a/cmds/telecom/src/com/android/commands/telecom/Telecom.java b/cmds/telecom/src/com/android/commands/telecom/Telecom.java
index 5f13a5ce3aae8..90c8c03585491 100644
--- a/cmds/telecom/src/com/android/commands/telecom/Telecom.java
+++ b/cmds/telecom/src/com/android/commands/telecom/Telecom.java
@@ -66,6 +66,7 @@ public final class Telecom extends BaseCommand {
     private static final String COMMAND_SET_PHONE_ACCOUNT_SUGGESTION_COMPONENT =
             "set-phone-acct-suggestion-component";
     private static final String COMMAND_UNREGISTER_PHONE_ACCOUNT = "unregister-phone-account";
+    private static final String COMMAND_SET_CALL_DIAGNOSTIC_SERVICE = "set-call-diagnostic-service";
     private static final String COMMAND_SET_DEFAULT_DIALER = "set-default-dialer";
     private static final String COMMAND_GET_DEFAULT_DIALER = "get-default-dialer";
     private static final String COMMAND_STOP_BLOCK_SUPPRESSION = "stop-block-suppression";
@@ -111,6 +112,7 @@ public final class Telecom extends BaseCommand {
                 + "usage: telecom register-sim-phone-account <COMPONENT> <ID> <USER_SN>"
                 + " <LABEL> <ADDRESS>\n"
                 + "usage: telecom unregister-phone-account <COMPONENT> <ID> <USER_SN>\n"
+                + "usage: telecom set-call-diagnostic-service <PACKAGE>\n"
                 + "usage: telecom set-default-dialer <PACKAGE>\n"
                 + "usage: telecom get-default-dialer\n"
                 + "usage: telecom get-system-dialer\n"
@@ -130,6 +132,7 @@ public final class Telecom extends BaseCommand {
                 + "telecom set-phone-account-disabled: Disables the given phone account, if it"
                         + " has already been registered with telecom.\n"
                 + "\n"
+                + "telecom set-call-diagnostic-service: overrides call diagnostic service.\n"
                 + "telecom set-default-dialer: Sets the override default dialer to the given"
                         + " component; this will override whatever the dialer role is set to.\n"
                 + "\n"
@@ -205,6 +208,9 @@ public final class Telecom extends BaseCommand {
             case COMMAND_SET_PHONE_ACCOUNT_SUGGESTION_COMPONENT:
                 runSetTestPhoneAcctSuggestionComponent();
                 break;
+            case COMMAND_SET_CALL_DIAGNOSTIC_SERVICE:
+                runSetCallDiagnosticService();
+                break;
             case COMMAND_REGISTER_SIM_PHONE_ACCOUNT:
                 runRegisterSimPhoneAccount();
                 break;
@@ -319,6 +325,13 @@ public final class Telecom extends BaseCommand {
         mTelecomService.addOrRemoveTestCallCompanionApp(packageName, isAddedBool);
     }
 
+    private void runSetCallDiagnosticService() throws RemoteException {
+        String packageName = nextArg();
+        if ("default".equals(packageName)) packageName = null;
+        mTelecomService.setTestCallDiagnosticService(packageName);
+        System.out.println("Success - " + packageName + " set as call diagnostic service.");
+    }
+
     private void runSetTestPhoneAcctSuggestionComponent() throws RemoteException {
         final String componentName = nextArg();
         mTelecomService.setTestPhoneAcctSuggestionComponent(componentName);
diff --git a/core/api/current.txt b/core/api/current.txt
index b0d659a6caa0c..af730596f4041 100644
--- a/core/api/current.txt
+++ b/core/api/current.txt
@@ -37886,6 +37886,10 @@ package android.telecom {
     method public void unhold();
     method public void unregisterCallback(android.telecom.Call.Callback);
     field @Deprecated public static final String AVAILABLE_PHONE_ACCOUNTS = "selectPhoneAccountAccounts";
+    field public static final String EVENT_CLEAR_DIAGNOSTIC_MESSAGE = "android.telecom.event.CLEAR_DIAGNOSTIC_MESSAGE";
+    field public static final String EVENT_DISPLAY_DIAGNOSTIC_MESSAGE = "android.telecom.event.DISPLAY_DIAGNOSTIC_MESSAGE";
+    field public static final String EXTRA_DIAGNOSTIC_MESSAGE = "android.telecom.extra.DIAGNOSTIC_MESSAGE";
+    field public static final String EXTRA_DIAGNOSTIC_MESSAGE_ID = "android.telecom.extra.DIAGNOSTIC_MESSAGE_ID";
     field public static final String EXTRA_LAST_EMERGENCY_CALLBACK_TIME_MILLIS = "android.telecom.extra.LAST_EMERGENCY_CALLBACK_TIME_MILLIS";
     field public static final String EXTRA_SILENT_RINGING_REQUESTED = "android.telecom.extra.SILENT_RINGING_REQUESTED";
     field public static final String EXTRA_SUGGESTED_PHONE_ACCOUNTS = "android.telecom.extra.SUGGESTED_PHONE_ACCOUNTS";
diff --git a/core/api/system-current.txt b/core/api/system-current.txt
index d2d301d26e120..483cc3147fa85 100644
--- a/core/api/system-current.txt
+++ b/core/api/system-current.txt
@@ -30,6 +30,7 @@ package android {
     field public static final String BACKUP = "android.permission.BACKUP";
     field public static final String BIND_ATTENTION_SERVICE = "android.permission.BIND_ATTENTION_SERVICE";
     field public static final String BIND_AUGMENTED_AUTOFILL_SERVICE = "android.permission.BIND_AUGMENTED_AUTOFILL_SERVICE";
+    field public static final String BIND_CALL_DIAGNOSTIC_SERVICE = "android.permission.BIND_CALL_DIAGNOSTIC_SERVICE";
     field public static final String BIND_CELL_BROADCAST_SERVICE = "android.permission.BIND_CELL_BROADCAST_SERVICE";
     field @Deprecated public static final String BIND_CONNECTION_SERVICE = "android.permission.BIND_CONNECTION_SERVICE";
     field public static final String BIND_CONTENT_CAPTURE_SERVICE = "android.permission.BIND_CONTENT_CAPTURE_SERVICE";
@@ -8943,6 +8944,16 @@ package android.telecom {
     ctor @Deprecated public Call.Listener();
   }
 
+  public abstract class CallDiagnosticService extends android.app.Service {
+    ctor public CallDiagnosticService();
+    method @Nullable public android.os.IBinder onBind(@NonNull android.content.Intent);
+    method public abstract void onBluetoothCallQualityReportReceived(@NonNull android.telecom.BluetoothCallQualityReport);
+    method public abstract void onCallAudioStateChanged(@NonNull android.telecom.CallAudioState);
+    method @NonNull public abstract android.telecom.DiagnosticCall onInitializeDiagnosticCall(@NonNull android.telecom.Call.Details);
+    method public abstract void onRemoveDiagnosticCall(@NonNull android.telecom.DiagnosticCall);
+    field public static final String SERVICE_INTERFACE = "android.telecom.CallDiagnosticService";
+  }
+
   public static class CallScreeningService.CallResponse.Builder {
     method @NonNull @RequiresPermission(android.Manifest.permission.CAPTURE_AUDIO_OUTPUT) public android.telecom.CallScreeningService.CallResponse.Builder setShouldScreenCallViaAudioProcessing(boolean);
   }
@@ -8974,6 +8985,9 @@ package android.telecom {
     method public void setTelecomCallId(@NonNull String);
     field public static final int CAPABILITY_CONFERENCE_HAS_NO_CHILDREN = 2097152; // 0x200000
     field public static final int CAPABILITY_SPEED_UP_MT_AUDIO = 262144; // 0x40000
+    field public static final String EVENT_DEVICE_TO_DEVICE_MESSAGE = "android.telecom.event.DEVICE_TO_DEVICE_MESSAGE";
+    field public static final String EXTRA_DEVICE_TO_DEVICE_MESSAGE_TYPE = "android.telecom.extra.DEVICE_TO_DEVICE_MESSAGE_TYPE";
+    field public static final String EXTRA_DEVICE_TO_DEVICE_MESSAGE_VALUE = "android.telecom.extra.DEVICE_TO_DEVICE_MESSAGE_VALUE";
     field public static final String EXTRA_DISABLE_ADD_CALL = "android.telecom.extra.DISABLE_ADD_CALL";
     field public static final int PROPERTY_EMERGENCY_CALLBACK_MODE = 1; // 0x1
     field public static final int PROPERTY_GENERIC_CONFERENCE = 2; // 0x2
@@ -8989,6 +9003,34 @@ package android.telecom {
     method public final void addExistingConnection(@NonNull android.telecom.PhoneAccountHandle, @NonNull android.telecom.Connection, @NonNull android.telecom.Conference);
   }
 
+  public abstract class DiagnosticCall {
+    ctor public DiagnosticCall();
+    method public final void clearDiagnosticMessage(int);
+    method public final void displayDiagnosticMessage(int, @NonNull CharSequence);
+    method @NonNull public android.telecom.Call.Details getCallDetails();
+    method public abstract void onCallDetailsChanged(@NonNull android.telecom.Call.Details);
+    method @Nullable public abstract CharSequence onCallDisconnected(int, int);
+    method @Nullable public abstract CharSequence onCallDisconnected(@NonNull android.telephony.ims.ImsReasonInfo);
+    method public abstract void onCallQualityReceived(@NonNull android.telephony.CallQuality);
+    method public abstract void onReceiveDeviceToDeviceMessage(int, int);
+    method public final void sendDeviceToDeviceMessage(int, int);
+    field public static final int AUDIO_CODEC_AMR_NB = 3; // 0x3
+    field public static final int AUDIO_CODEC_AMR_WB = 2; // 0x2
+    field public static final int AUDIO_CODEC_EVS = 1; // 0x1
+    field public static final int BATTERY_STATE_CHARGING = 3; // 0x3
+    field public static final int BATTERY_STATE_GOOD = 2; // 0x2
+    field public static final int BATTERY_STATE_LOW = 1; // 0x1
+    field public static final int COVERAGE_GOOD = 2; // 0x2
+    field public static final int COVERAGE_POOR = 1; // 0x1
+    field public static final int MESSAGE_CALL_AUDIO_CODEC = 2; // 0x2
+    field public static final int MESSAGE_CALL_NETWORK_TYPE = 1; // 0x1
+    field public static final int MESSAGE_DEVICE_BATTERY_STATE = 3; // 0x3
+    field public static final int MESSAGE_DEVICE_NETWORK_COVERAGE = 4; // 0x4
+    field public static final int NETWORK_TYPE_IWLAN = 2; // 0x2
+    field public static final int NETWORK_TYPE_LTE = 1; // 0x1
+    field public static final int NETWORK_TYPE_NR = 3; // 0x3
+  }
+
   public abstract class InCallService extends android.app.Service {
     method @Deprecated public android.telecom.Phone getPhone();
     method @Deprecated public void onPhoneCreated(android.telecom.Phone);
diff --git a/core/res/AndroidManifest.xml b/core/res/AndroidManifest.xml
index 8ed7a5c2611e5..298a6c5005e13 100644
--- a/core/res/AndroidManifest.xml
+++ b/core/res/AndroidManifest.xml
@@ -2268,6 +2268,15 @@
     <permission android:name="android.permission.BIND_PHONE_ACCOUNT_SUGGESTION_SERVICE"
         android:protectionLevel="signature" />
 
+    <!-- Must be required by a {@link android.telecom.CallDiagnosticService},
+         to ensure that only the system can bind to it.
+         <p>Protection level: signature
+         @SystemApi
+         @hide
+    -->
+    <permission android:name="android.permission.BIND_CALL_DIAGNOSTIC_SERVICE"
+        android:protectionLevel="signature" />
+
     <!-- Must be required by a {@link android.telecom.CallRedirectionService},
          to ensure that only the system can bind to it.
          <p>Protection level: signature|privileged
diff --git a/telecomm/java/android/telecom/BluetoothCallQualityReport.aidl b/telecomm/java/android/telecom/BluetoothCallQualityReport.aidl
new file mode 100644
index 0000000000000..685fe9c927b13
--- /dev/null
+++ b/telecomm/java/android/telecom/BluetoothCallQualityReport.aidl
@@ -0,0 +1,22 @@
+/*
+ * Copyright 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 android.telecom;
+
+/**
+ * {@hide}
+ */
+parcelable BluetoothCallQualityReport;
diff --git a/telecomm/java/android/telecom/BluetoothCallQualityReport.java b/telecomm/java/android/telecom/BluetoothCallQualityReport.java
index 10339a818205f..8703d84831ffc 100644
--- a/telecomm/java/android/telecom/BluetoothCallQualityReport.java
+++ b/telecomm/java/android/telecom/BluetoothCallQualityReport.java
@@ -24,6 +24,8 @@ import android.os.Bundle;
 import android.os.Parcel;
 import android.os.Parcelable;
 
+import java.util.Objects;
+
 /**
  * This class represents the quality report that bluetooth framework sends
  * whenever there's a bad voice quality is detected from their side.
@@ -145,6 +147,26 @@ public final class BluetoothCallQualityReport implements Parcelable {
                 }
             };
 
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        BluetoothCallQualityReport that = (BluetoothCallQualityReport) o;
+        return mSentTimestampMillis == that.mSentTimestampMillis
+                && mChoppyVoice == that.mChoppyVoice && mRssiDbm == that.mRssiDbm
+                && mSnrDb == that.mSnrDb
+                && mRetransmittedPacketsCount == that.mRetransmittedPacketsCount
+                && mPacketsNotReceivedCount == that.mPacketsNotReceivedCount
+                && mNegativeAcknowledgementCount == that.mNegativeAcknowledgementCount;
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(mSentTimestampMillis, mChoppyVoice, mRssiDbm, mSnrDb,
+                mRetransmittedPacketsCount, mPacketsNotReceivedCount,
+                mNegativeAcknowledgementCount);
+    }
+
     /**
      * Builder class for {@link ConnectionRequest}
      */
diff --git a/telecomm/java/android/telecom/Call.java b/telecomm/java/android/telecom/Call.java
index 1238e7b69a872..4679b9c497bdc 100755
--- a/telecomm/java/android/telecom/Call.java
+++ b/telecomm/java/android/telecom/Call.java
@@ -267,6 +267,64 @@ public final class Call {
     public static final String EVENT_HANDOVER_FAILED =
             "android.telecom.event.HANDOVER_FAILED";
 
+    /**
+     * Event reported from the Telecom stack to report an in-call diagnostic message which the
+     * dialer app may opt to display to the user.  A diagnostic message is used to communicate
+     * scenarios the device has detected which may impact the quality of the ongoing call.
+     * <p>
+     * For example a problem with a bluetooth headset may generate a recommendation for the user to
+     * try using the speakerphone instead, or if the device detects it has entered a poor service
+     * area, the user might be warned so that they can finish their call prior to it dropping.
+     * <p>
+     * A diagnostic message is considered persistent in nature.  When the user enters a poor service
+     * area, for example, the accompanying diagnostic message persists until they leave the area
+     * of poor service.  Each message is accompanied with a {@link #EXTRA_DIAGNOSTIC_MESSAGE_ID}
+     * which uniquely identifies the diagnostic condition being reported.  The framework raises a
+     * call event of type {@link #EVENT_CLEAR_DIAGNOSTIC_MESSAGE} when the condition reported has
+     * been cleared.  The dialer app should display the diagnostic message until it is cleared.
+     * If multiple diagnostic messages are sent with different IDs (which have not yet been cleared)
+     * the dialer app should prioritize the most recently received message, but still provide the
+     * user with a means to review past messages.
+     * <p>
+     * The text of the message is found in {@link #EXTRA_DIAGNOSTIC_MESSAGE} in the form of a human
+     * readable {@link CharSequence} which is intended for display in the call UX.
+     * <p>
+     * The telecom framework audibly notifies the user of the presence of a diagnostic message, so
+     * the dialer app needs only to concern itself with visually displaying the message.
+     * <p>
+     * The dialer app receives this event via
+     * {@link Call.Callback#onConnectionEvent(Call, String, Bundle)}.
+     */
+    public static final String EVENT_DISPLAY_DIAGNOSTIC_MESSAGE =
+            "android.telecom.event.DISPLAY_DIAGNOSTIC_MESSAGE";
+
+    /**
+     * Event reported from the telecom framework when a diagnostic message previously raised with
+     * {@link #EVENT_DISPLAY_DIAGNOSTIC_MESSAGE} has cleared and is no longer pertinent.
+     * <p>
+     * The {@link #EXTRA_DIAGNOSTIC_MESSAGE_ID} indicates the diagnostic message which has been
+     * cleared.
+     * <p>
+     * The dialer app receives this event via
+     * {@link Call.Callback#onConnectionEvent(Call, String, Bundle)}.
+     */
+    public static final String EVENT_CLEAR_DIAGNOSTIC_MESSAGE =
+            "android.telecom.event.CLEAR_DIAGNOSTIC_MESSAGE";
+
+    /**
+     * Integer extra representing a message ID for a message posted via
+     * {@link #EVENT_DISPLAY_DIAGNOSTIC_MESSAGE}.  Used to ensure that the dialer app knows when
+     * the message in question has cleared via {@link #EVENT_CLEAR_DIAGNOSTIC_MESSAGE}.
+     */
+    public static final String EXTRA_DIAGNOSTIC_MESSAGE_ID =
+            "android.telecom.extra.DIAGNOSTIC_MESSAGE_ID";
+
+    /**
+     * {@link CharSequence} extra used with {@link #EVENT_DISPLAY_DIAGNOSTIC_MESSAGE}.  This is the
+     * diagnostic message the dialer app should display.
+     */
+    public static final String EXTRA_DIAGNOSTIC_MESSAGE =
+            "android.telecom.extra.DIAGNOSTIC_MESSAGE";
 
     /**
      * Reject reason used with {@link #reject(int)} to indicate that the user is rejecting this
diff --git a/telecomm/java/android/telecom/CallDiagnosticService.java b/telecomm/java/android/telecom/CallDiagnosticService.java
new file mode 100644
index 0000000000000..201c5db74e16d
--- /dev/null
+++ b/telecomm/java/android/telecom/CallDiagnosticService.java
@@ -0,0 +1,328 @@
+/*
+ * 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 android.telecom;
+
+import android.annotation.NonNull;
+import android.annotation.Nullable;
+import android.annotation.SdkConstant;
+import android.annotation.SystemApi;
+import android.app.Service;
+import android.content.Intent;
+import android.os.IBinder;
+import android.os.RemoteException;
+import android.util.ArrayMap;
+
+import com.android.internal.telecom.ICallDiagnosticService;
+import com.android.internal.telecom.ICallDiagnosticServiceAdapter;
+
+import java.util.Map;
+
+/**
+ * The platform supports a single OEM provided {@link CallDiagnosticService}, as defined by the
+ * {@code call_diagnostic_service_package_name} key in the
+ * {@code packages/services/Telecomm/res/values/config.xml} file.  An OEM can use this API to help
+ * provide more actionable information about calling issues the user encounters during and after
+ * a call.
+ *
+ * <h1>Manifest Declaration</h1>
+ * The following is an example of how to declare the service entry in the
+ * {@link CallDiagnosticService} manifest file:
+ * <pre>
+ * {@code
+ * <service android:name="your.package.YourCallDiagnosticServiceImplementation"
+ *          android:permission="android.permission.BIND_CALL_DIAGNOSTIC_SERVICE">
+ *      <intent-filter>
+ *          <action android:name="android.telecom.CallDiagnosticService"/>
+ *      </intent-filter>
+ * </service>
+ * }
+ * </pre>
+ * @hide
+ */
+@SystemApi
+public abstract class CallDiagnosticService extends Service {
+
+    /**
+     * Binder stub implementation which handles incoming requests from Telecom.
+     */
+    private final class CallDiagnosticServiceBinder extends ICallDiagnosticService.Stub {
+
+        @Override
+        public void setAdapter(ICallDiagnosticServiceAdapter adapter) throws RemoteException {
+            handleSetAdapter(adapter);
+        }
+
+        @Override
+        public void initializeDiagnosticCall(ParcelableCall call) throws RemoteException {
+            handleCallAdded(call);
+        }
+
+        @Override
+        public void updateCall(ParcelableCall call) throws RemoteException {
+            handleCallUpdated(call);
+        }
+
+        @Override
+        public void removeDiagnosticCall(String callId) throws RemoteException {
+            handleCallRemoved(callId);
+        }
+
+        @Override
+        public void updateCallAudioState(CallAudioState callAudioState) throws RemoteException {
+            onCallAudioStateChanged(callAudioState);
+        }
+
+        @Override
+        public void receiveDeviceToDeviceMessage(String callId, int message, int value) {
+            handleReceivedD2DMessage(callId, message, value);
+        }
+
+        @Override
+        public void receiveBluetoothCallQualityReport(BluetoothCallQualityReport qualityReport)
+                throws RemoteException {
+            handleBluetoothCallQualityReport(qualityReport);
+        }
+    }
+
+    /**
+     * Listens to events raised by a {@link DiagnosticCall}.
+     */
+    private android.telecom.DiagnosticCall.Listener mDiagnosticCallListener =
+            new android.telecom.DiagnosticCall.Listener() {
+
+                @Override
+                public void onSendDeviceToDeviceMessage(DiagnosticCall diagnosticCall,
+                        @DiagnosticCall.MessageType int message, int value) {
+                    handleSendDeviceToDeviceMessage(diagnosticCall, message, value);
+                }
+
+                @Override
+                public void onDisplayDiagnosticMessage(DiagnosticCall diagnosticCall, int messageId,
+                        CharSequence message) {
+                    handleDisplayDiagnosticMessage(diagnosticCall, messageId, message);
+                }
+
+                @Override
+                public void onClearDiagnosticMessage(DiagnosticCall diagnosticCall, int messageId) {
+                    handleClearDiagnosticMessage(diagnosticCall, messageId);
+                }
+            };
+
+    /**
+     * The {@link Intent} that must be declared as handled by the service.
+     */
+    @SdkConstant(SdkConstant.SdkConstantType.SERVICE_ACTION)
+    public static final String SERVICE_INTERFACE = "android.telecom.CallDiagnosticService";
+
+    /**
+     * Map which tracks the Telecom calls received from the Telecom stack.
+     */
+    private final Map<String, Call.Details> mCallByTelecomCallId = new ArrayMap<>();
+    private final Map<String, DiagnosticCall> mDiagnosticCallByTelecomCallId = new ArrayMap<>();
+    private ICallDiagnosticServiceAdapter mAdapter;
+
+    @Nullable
+    @Override
+    public IBinder onBind(@NonNull Intent intent) {
+        Log.i(this, "onBind!");
+        return new CallDiagnosticServiceBinder();
+    }
+
+    /**
+     * Telecom calls this method on the {@link CallDiagnosticService} with details about a new call
+     * which was added to Telecom.
+     * <p>
+     * The {@link CallDiagnosticService} returns an implementation of {@link DiagnosticCall} to be
+     * used for the lifespan of this call.
+     *
+     * @param call The details of the new call.
+     * @return An instance of {@link DiagnosticCall} which the {@link CallDiagnosticService}
+     * provides to be used for the lifespan of the call.
+     * @throws IllegalArgumentException if a {@code null} {@link DiagnosticCall} is returned.
+     */
+    public abstract @NonNull DiagnosticCall onInitializeDiagnosticCall(@NonNull
+            android.telecom.Call.Details call);
+
+    /**
+     * Telecom calls this method when a previous created {@link DiagnosticCall} is no longer needed.
+     * This happens when Telecom is no longer tracking the call in question.
+     * @param call The diagnostic call which is no longer tracked by Telecom.
+     */
+    public abstract void onRemoveDiagnosticCall(@NonNull DiagnosticCall call);
+
+    /**
+     * Telecom calls this method when the audio routing or available audio route information
+     * changes.
+     * <p>
+     * Audio state is common to all calls.
+     *
+     * @param audioState The new audio state.
+     */
+    public abstract void onCallAudioStateChanged(
+            @NonNull CallAudioState audioState);
+
+    /**
+     * Telecom calls this method when a {@link BluetoothCallQualityReport} is received from the
+     * bluetooth stack.
+     * @param qualityReport the {@link BluetoothCallQualityReport}.
+     */
+    public abstract void onBluetoothCallQualityReportReceived(
+            @NonNull BluetoothCallQualityReport qualityReport);
+
+    /**
+     * Handles a request from Telecom to set the adapater used to communicate back to Telecom.
+     * @param adapter
+     */
+    private void handleSetAdapter(@NonNull ICallDiagnosticServiceAdapter adapter) {
+        mAdapter = adapter;
+    }
+
+    /**
+     * Handles a request from Telecom to add a new call.
+     * @param parcelableCall
+     */
+    private void handleCallAdded(@NonNull ParcelableCall parcelableCall) {
+        String telecomCallId = parcelableCall.getId();
+        Log.i(this, "handleCallAdded: callId=%s - added", telecomCallId);
+        Call.Details newCallDetails = Call.Details.createFromParcelableCall(parcelableCall);
+        mCallByTelecomCallId.put(telecomCallId, newCallDetails);
+
+        DiagnosticCall diagnosticCall = onInitializeDiagnosticCall(newCallDetails);
+        if (diagnosticCall == null) {
+            throw new IllegalArgumentException("A valid DiagnosticCall instance was not provided.");
+        }
+        diagnosticCall.setListener(mDiagnosticCallListener);
+        diagnosticCall.setCallId(telecomCallId);
+        mDiagnosticCallByTelecomCallId.put(telecomCallId, diagnosticCall);
+    }
+
+    /**
+     * Handles an update to {@link Call.Details} notified by Telecom.
+     * Caches the call details and notifies the {@link DiagnosticCall} of the change via
+     * {@link DiagnosticCall#onCallDetailsChanged(Call.Details)}.
+     * @param parcelableCall the new parceled call details from Telecom.
+     */
+    private void handleCallUpdated(@NonNull ParcelableCall parcelableCall) {
+        String telecomCallId = parcelableCall.getId();
+        Log.i(this, "handleCallUpdated: callId=%s - updated", telecomCallId);
+        Call.Details newCallDetails = Call.Details.createFromParcelableCall(parcelableCall);
+
+        DiagnosticCall diagnosticCall = mDiagnosticCallByTelecomCallId.get(telecomCallId);
+        mCallByTelecomCallId.put(telecomCallId, newCallDetails);
+        diagnosticCall.handleCallUpdated(newCallDetails);
+    }
+
+    /**
+     * Handles a request from Telecom to remove an existing call.
+     * @param telecomCallId
+     */
+    private void handleCallRemoved(@NonNull String telecomCallId) {
+        Log.i(this, "handleCallRemoved: callId=%s - removed", telecomCallId);
+
+        if (mCallByTelecomCallId.containsKey(telecomCallId)) {
+            mCallByTelecomCallId.remove(telecomCallId);
+        }
+        if (mDiagnosticCallByTelecomCallId.containsKey(telecomCallId)) {
+            DiagnosticCall call = mDiagnosticCallByTelecomCallId.remove(telecomCallId);
+            // Inform the service of the removed call.
+            onRemoveDiagnosticCall(call);
+        }
+    }
+
+    /**
+     * Handles an incoming device to device message received from Telecom.  Notifies the
+     * {@link DiagnosticCall} via {@link DiagnosticCall#onReceiveDeviceToDeviceMessage(int, int)}.
+     * @param callId
+     * @param message
+     * @param value
+     */
+    private void handleReceivedD2DMessage(@NonNull String callId, int message, int value) {
+        Log.i(this, "handleReceivedD2DMessage: callId=%s, msg=%d/%d", callId, message, value);
+        DiagnosticCall diagnosticCall = mDiagnosticCallByTelecomCallId.get(callId);
+        diagnosticCall.onReceiveDeviceToDeviceMessage(message, value);
+    }
+
+    /**
+     * Handles an incoming bluetooth call quality report from Telecom.  Notifies via
+     * {@link CallDiagnosticService#onBluetoothCallQualityReportReceived(
+     * BluetoothCallQualityReport)}.
+     * @param qualityReport The bluetooth call quality remote.
+     */
+    private void handleBluetoothCallQualityReport(@NonNull BluetoothCallQualityReport
+            qualityReport) {
+        Log.i(this, "handleBluetoothCallQualityReport; report=%s", qualityReport);
+        onBluetoothCallQualityReportReceived(qualityReport);
+    }
+
+    /**
+     * Handles a request from a {@link DiagnosticCall} to send a device to device message (received
+     * via {@link DiagnosticCall#sendDeviceToDeviceMessage(int, int)}.
+     * @param diagnosticCall
+     * @param message
+     * @param value
+     */
+    private void handleSendDeviceToDeviceMessage(@NonNull DiagnosticCall diagnosticCall,
+            int message, int value) {
+        String callId = diagnosticCall.getCallId();
+        try {
+            mAdapter.sendDeviceToDeviceMessage(callId, message, value);
+            Log.i(this, "handleSendDeviceToDeviceMessage: call=%s; msg=%d/%d", callId, message,
+                    value);
+        } catch (RemoteException e) {
+            Log.w(this, "handleSendDeviceToDeviceMessage: call=%s; msg=%d/%d failed %s",
+                    callId, message, value, e);
+        }
+    }
+
+    /**
+     * Handles a request from a {@link DiagnosticCall} to display an in-call diagnostic message.
+     * Originates from {@link DiagnosticCall#displayDiagnosticMessage(int, CharSequence)}.
+     * @param diagnosticCall
+     * @param messageId
+     * @param message
+     */
+    private void handleDisplayDiagnosticMessage(DiagnosticCall diagnosticCall, int messageId,
+            CharSequence message) {
+        String callId = diagnosticCall.getCallId();
+        try {
+            mAdapter.displayDiagnosticMessage(callId, messageId, message);
+            Log.i(this, "handleDisplayDiagnosticMessage: call=%s; msg=%d/%s", callId, messageId,
+                    message);
+        } catch (RemoteException e) {
+            Log.w(this, "handleDisplayDiagnosticMessage: call=%s; msg=%d/%s failed %s",
+                    callId, messageId, message, e);
+        }
+    }
+
+    /**
+     * Handles a request from a {@link DiagnosticCall} to clear a previously shown diagnostic
+     * message.
+     * Originates from {@link DiagnosticCall#clearDiagnosticMessage(int)}.
+     * @param diagnosticCall
+     * @param messageId
+     */
+    private void handleClearDiagnosticMessage(DiagnosticCall diagnosticCall, int messageId) {
+        String callId = diagnosticCall.getCallId();
+        try {
+            mAdapter.clearDiagnosticMessage(callId, messageId);
+            Log.i(this, "handleClearDiagnosticMessage: call=%s; msg=%d", callId, messageId);
+        } catch (RemoteException e) {
+            Log.w(this, "handleClearDiagnosticMessage: call=%s; msg=%d failed %s",
+                    callId, messageId, e);
+        }
+    }
+}
diff --git a/telecomm/java/android/telecom/Connection.java b/telecomm/java/android/telecom/Connection.java
index 335a10221d061..973b20ad0713c 100644
--- a/telecomm/java/android/telecom/Connection.java
+++ b/telecomm/java/android/telecom/Connection.java
@@ -926,6 +926,46 @@ public abstract class Connection extends Conferenceable {
     public static final String EVENT_RTT_AUDIO_INDICATION_CHANGED =
             "android.telecom.event.RTT_AUDIO_INDICATION_CHANGED";
 
+    /**
+     * Connection event used to signal between the telephony {@link ConnectionService} and Telecom
+     * when device to device messages are sent/received.
+     * <p>
+     * Device to device messages originating from the network are sent by telephony using
+     * {@link Connection#sendConnectionEvent(String, Bundle)} and are routed up to any active
+     * {@link CallDiagnosticService} implementation which is active.
+     * <p>
+     * Likewise, if a {@link CallDiagnosticService} sends a message using
+     * {@link DiagnosticCall#sendDeviceToDeviceMessage(int, int)}, it will be routed to telephony
+     * via {@link Connection#onCallEvent(String, Bundle)}.  The telephony stack will relay the
+     * message to the other device.
+     * @hide
+     */
+    @SystemApi
+    public static final String EVENT_DEVICE_TO_DEVICE_MESSAGE =
+            "android.telecom.event.DEVICE_TO_DEVICE_MESSAGE";
+
+    /**
+     * Sent along with {@link #EVENT_DEVICE_TO_DEVICE_MESSAGE} to indicate the device to device
+     * message type.
+     *
+     * See {@link DiagnosticCall} for more information.
+     * @hide
+     */
+    @SystemApi
+    public static final String EXTRA_DEVICE_TO_DEVICE_MESSAGE_TYPE =
+            "android.telecom.extra.DEVICE_TO_DEVICE_MESSAGE_TYPE";
+
+    /**
+     * Sent along with {@link #EVENT_DEVICE_TO_DEVICE_MESSAGE} to indicate the device to device
+     * message value.
+     * <p>
+     * See {@link DiagnosticCall} for more information.
+     * @hide
+     */
+    @SystemApi
+    public static final String EXTRA_DEVICE_TO_DEVICE_MESSAGE_VALUE =
+            "android.telecom.extra.DEVICE_TO_DEVICE_MESSAGE_VALUE";
+
     // Flag controlling whether PII is emitted into the logs
     private static final boolean PII_DEBUG = Log.isLoggable(android.util.Log.DEBUG);
 
diff --git a/telecomm/java/android/telecom/DiagnosticCall.java b/telecomm/java/android/telecom/DiagnosticCall.java
new file mode 100644
index 0000000000000..a4952899eb465
--- /dev/null
+++ b/telecomm/java/android/telecom/DiagnosticCall.java
@@ -0,0 +1,381 @@
+/*
+ * 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 android.telecom;
+
+import android.annotation.IntDef;
+import android.annotation.NonNull;
+import android.annotation.Nullable;
+import android.annotation.SystemApi;
+import android.telephony.Annotation;
+import android.telephony.CallQuality;
+import android.telephony.ims.ImsReasonInfo;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+
+/**
+ * A {@link DiagnosticCall} provides a way for a {@link CallDiagnosticService} to receive diagnostic
+ * information about a mobile call on the device.  The {@link CallDiagnosticService} can generate
+ * mid-call diagnostic messages using the {@link #displayDiagnosticMessage(int, CharSequence)} API
+ * which provides the user with valuable information about conditions impacting their call and
+ * corrective actions.  For example, if the {@link CallDiagnosticService} determines that conditions
+ * on the call are degrading, it can inform the user that the call may soon drop and that they
+ * can try using a different calling method (e.g. VOIP or WIFI).
+ * @hide
+ */
+@SystemApi
+public abstract class DiagnosticCall {
+
+    /**
+     * @hide
+     */
+    public interface Listener {
+        void onSendDeviceToDeviceMessage(DiagnosticCall diagnosticCall, int message, int value);
+        void onDisplayDiagnosticMessage(DiagnosticCall diagnosticCall, int messageId,
+                CharSequence message);
+        void onClearDiagnosticMessage(DiagnosticCall diagnosticCall, int messageId);
+    }
+
+    /**
+     * Device to device message sent via {@link #sendDeviceToDeviceMessage(int, int)} (received via
+     * {@link #onReceiveDeviceToDeviceMessage(int, int)}) which communicates the radio access type
+     * used for the current call.  Based loosely on the
+     * {@link android.telephony.TelephonyManager#getNetworkType(int)} for the call, provides a
+     * high level summary of the call radio access type.
+     * <p>
+     * Valid values:
+     * <UL>
+     *     <LI>{@link #NETWORK_TYPE_LTE}</LI>
+     *     <LI>{@link #NETWORK_TYPE_IWLAN}</LI>
+     *     <LI>{@link #NETWORK_TYPE_NR}</LI>
+     * </UL>
+     */
+    public static final int MESSAGE_CALL_NETWORK_TYPE = 1;
+
+    /**
+     * Device to device message sent via {@link #sendDeviceToDeviceMessage(int, int)} (received via
+     * {@link #onReceiveDeviceToDeviceMessage(int, int)}) which communicates the call audio codec
+     * used for the current call.  Based loosely on the {@link Connection#EXTRA_AUDIO_CODEC} for a
+     * call.
+     * <p>
+     * Valid values:
+     * <UL>
+     *     <LI>{@link #AUDIO_CODEC_EVS}</LI>
+     *     <LI>{@link #AUDIO_CODEC_AMR_WB}</LI>
+     *     <LI>{@link #AUDIO_CODEC_AMR_NB}</LI>
+     * </UL>
+     */
+    public static final int MESSAGE_CALL_AUDIO_CODEC = 2;
+
+    /**
+     * Device to device message sent via {@link #sendDeviceToDeviceMessage(int, int)} (received via
+     * {@link #onReceiveDeviceToDeviceMessage(int, int)}) which communicates the battery state of
+     * the device.  Will typically mirror battery state reported via intents such as
+     * {@link android.content.Intent#ACTION_BATTERY_LOW}.
+     * <p>
+     * Valid values:
+     * <UL>
+     *     <LI>{@link #BATTERY_STATE_LOW}</LI>
+     *     <LI>{@link #BATTERY_STATE_GOOD}</LI>
+     *     <LI>{@link #BATTERY_STATE_CHARGING}</LI>
+     * </UL>
+     */
+    public static final int MESSAGE_DEVICE_BATTERY_STATE = 3;
+
+    /**
+     * Device to device message sent via {@link #sendDeviceToDeviceMessage(int, int)} (received via
+     * {@link #onReceiveDeviceToDeviceMessage(int, int)}) which communicates the overall network
+     * coverage as it pertains to the current call.  A {@link CallDiagnosticService} should signal
+     * poor coverage if the network coverage reaches a level where there is a high probability of
+     * the call dropping as a result.
+     * <p>
+     * Valid values:
+     * <UL>
+     *     <LI>{@link #COVERAGE_POOR}</LI>
+     *     <LI>{@link #COVERAGE_GOOD}</LI>
+     * </UL>
+     */
+    public static final int MESSAGE_DEVICE_NETWORK_COVERAGE = 4;
+
+    /**@hide*/
+    @Retention(RetentionPolicy.SOURCE)
+    @IntDef(prefix = "MESSAGE_", value = {
+            MESSAGE_CALL_NETWORK_TYPE,
+            MESSAGE_CALL_AUDIO_CODEC,
+            MESSAGE_DEVICE_BATTERY_STATE,
+            MESSAGE_DEVICE_NETWORK_COVERAGE
+    })
+    public @interface MessageType {}
+
+    /**
+     * Used with {@link #MESSAGE_CALL_NETWORK_TYPE} to indicate an LTE network is being used for the
+     * call.
+     */
+    public static final int NETWORK_TYPE_LTE = 1;
+
+    /**
+     * Used with {@link #MESSAGE_CALL_NETWORK_TYPE} to indicate WIFI calling is in use for the call.
+     */
+    public static final int NETWORK_TYPE_IWLAN = 2;
+
+    /**
+     * Used with {@link #MESSAGE_CALL_NETWORK_TYPE} to indicate a 5G NR (new radio) network is in
+     * used for the call.
+     */
+    public static final int NETWORK_TYPE_NR = 3;
+
+    /**
+     * Used with {@link #MESSAGE_CALL_AUDIO_CODEC} to indicate call audio is using the
+     * Enhanced Voice Services (EVS) codec for the call.
+     */
+    public static final int AUDIO_CODEC_EVS = 1;
+
+    /**
+     * Used with {@link #MESSAGE_CALL_AUDIO_CODEC} to indicate call audio is using the AMR
+     * (adaptive multi-rate) WB (wide band) audio codec.
+     */
+    public static final int AUDIO_CODEC_AMR_WB = 2;
+
+    /**
+     * Used with {@link #MESSAGE_CALL_AUDIO_CODEC} to indicate call audio is using the AMR
+     * (adaptive multi-rate) NB (narrow band) audio codec.
+     */
+    public static final int AUDIO_CODEC_AMR_NB = 3;
+
+    /**
+     * Used with {@link #MESSAGE_DEVICE_BATTERY_STATE} to indicate that the battery is low.
+     */
+    public static final int BATTERY_STATE_LOW = 1;
+
+    /**
+     * Used with {@link #MESSAGE_DEVICE_BATTERY_STATE} to indicate that the battery is not low.
+     */
+    public static final int BATTERY_STATE_GOOD = 2;
+
+    /**
+     * Used with {@link #MESSAGE_DEVICE_BATTERY_STATE} to indicate that the battery is charging.
+     */
+    public static final int BATTERY_STATE_CHARGING = 3;
+
+    /**
+     * Used with {@link #MESSAGE_DEVICE_NETWORK_COVERAGE} to indicate that the coverage is poor.
+     */
+    public static final int COVERAGE_POOR = 1;
+
+    /**
+     * Used with {@link #MESSAGE_DEVICE_NETWORK_COVERAGE} to indicate that the coverage is good.
+     */
+    public static final int COVERAGE_GOOD = 2;
+
+    private Listener mListener;
+    private String mCallId;
+    private Call.Details mCallDetails;
+
+    /**
+     * @hide
+     */
+    public void setListener(@NonNull Listener listener) {
+        mListener = listener;
+    }
+
+    /**
+     * Sets the call ID for this {@link DiagnosticCall}.
+     * @param callId
+     * @hide
+     */
+    public void setCallId(@NonNull String callId) {
+        mCallId = callId;
+    }
+
+    /**
+     * @return the Telecom call ID for this {@link DiagnosticCall}.
+     * @hide
+     */
+    public @NonNull String getCallId() {
+        return mCallId;
+    }
+
+    /**
+     * Returns the latest {@link Call.Details} associated with this {@link DiagnosticCall} as
+     * reported by {@link #onCallDetailsChanged(Call.Details)}.
+     * @return The latest {@link Call.Details}.
+     */
+    public @NonNull Call.Details getCallDetails() {
+        return mCallDetails;
+    }
+
+    /**
+     * Telecom calls this method when the details of a call changes.
+     */
+    public abstract void onCallDetailsChanged(@NonNull android.telecom.Call.Details details);
+
+    /**
+     * The {@link CallDiagnosticService} implements this method to handle messages received via
+     * device to device communication.
+     * <p>
+     * See {@link #sendDeviceToDeviceMessage(int, int)} for background on device to device
+     * communication.
+     * <p>
+     * The underlying device to device communication protocol assumes that where there the two
+     * devices communicating are using a different version of the protocol, messages the recipient
+     * are not aware of are silently discarded.  This means an older client talking to a new client
+     * will not receive newer messages and values sent by the new client.
+     */
+    public abstract void onReceiveDeviceToDeviceMessage(
+            @MessageType int message,
+            int value);
+
+    /**
+     * Sends a device to device message to the device on the other end of this call.
+     * <p>
+     * Device to device communication is an Android platform feature which supports low bandwidth
+     * communication between Android devices while they are in a call.  The device to device
+     * communication leverages DTMF tones or RTP header extensions to pass messages.  The
+     * messages are unacknowledged and sent in a best-effort manner.  The protocols assume that the
+     * nature of the message are informational only and are used only to convey basic state
+     * information between devices.
+     * <p>
+     * Device to device messages are intentional simplifications of more rich indicators in the
+     * platform due to the extreme bandwidth constraints inherent with underlying device to device
+     * communication transports used by the telephony framework.  Device to device communication is
+     * either accomplished by adding RFC8285 compliant RTP header extensions to the audio packets
+     * for a call, or using the DTMF digits A-D as a communication pathway.  Signalling requirements
+     * for DTMF digits place a significant limitation on the amount of information which can be
+     * communicated during a call.
+     * <p>
+     * Allowed message types and values are:
+     * <ul>
+     *     <li>{@link #MESSAGE_CALL_NETWORK_TYPE}
+     *         <ul>
+     *             <li>{@link #NETWORK_TYPE_LTE}</li>
+     *             <li>{@link #NETWORK_TYPE_IWLAN}</li>
+     *             <li>{@link #NETWORK_TYPE_NR}</li>
+     *         </ul>
+     *     </li>
+     *     <li>{@link #MESSAGE_CALL_AUDIO_CODEC}
+     *         <ul>
+     *             <li>{@link #AUDIO_CODEC_EVS}</li>
+     *             <li>{@link #AUDIO_CODEC_AMR_WB}</li>
+     *             <li>{@link #AUDIO_CODEC_AMR_NB}</li>
+     *         </ul>
+     *     </li>
+     *     <li>{@link #MESSAGE_DEVICE_BATTERY_STATE}
+     *         <ul>
+     *             <li>{@link #BATTERY_STATE_LOW}</li>
+     *             <li>{@link #BATTERY_STATE_GOOD}</li>
+     *             <li>{@link #BATTERY_STATE_CHARGING}</li>
+     *         </ul>
+     *     </li>
+     *     <li>{@link #MESSAGE_DEVICE_NETWORK_COVERAGE}
+     *         <ul>
+     *             <li>{@link #COVERAGE_POOR}</li>
+     *             <li>{@link #COVERAGE_GOOD}</li>
+     *         </ul>
+     *     </li>
+     * </ul>
+     * @param message The message type to send.
+     * @param value The message value corresponding to the type.
+     */
+    public final void sendDeviceToDeviceMessage(int message, int value) {
+        if (mListener != null) {
+            mListener.onSendDeviceToDeviceMessage(this, message, value);
+        }
+    }
+
+    /**
+     * Telecom calls this method when a GSM or CDMA call disconnects.
+     * The CallDiagnosticService can return a human readable disconnect message which will be passed
+     * to the Dialer app as the {@link DisconnectCause#getDescription()}.  A dialer app typically
+     * shows this message at the termination of the call.  If {@code null} is returned, the
+     * disconnect message generated by the telephony stack will be shown instead.
+     * <p>
+     * @param disconnectCause the disconnect cause for the call.
+     * @param preciseDisconnectCause the precise disconnect cause for the call.
+     * @return the disconnect message to use in place of the default Telephony message, or
+     * {@code null} if the default message will not be overridden.
+     */
+    // TODO: Wire in Telephony support for this.
+    public abstract @Nullable CharSequence onCallDisconnected(
+            @Annotation.DisconnectCauses int disconnectCause,
+            @Annotation.PreciseDisconnectCauses int preciseDisconnectCause);
+
+    /**
+     * Telecom calls this method when an IMS call disconnects and Telephony has already
+     * provided the disconnect reason info and disconnect message for the call.  The
+     * {@link CallDiagnosticService} can intercept the raw IMS disconnect reason at this point and
+     * combine it with other call diagnostic information it is aware of to override the disconnect
+     * call message if desired.
+     *
+     * @param disconnectReason The {@link ImsReasonInfo} associated with the call disconnection.
+     * @return A user-readable call disconnect message to use in place of the platform-generated
+     * disconnect message, or {@code null} if the disconnect message should not be overridden.
+     */
+    // TODO: Wire in Telephony support for this.
+    public abstract @Nullable CharSequence onCallDisconnected(
+            @NonNull ImsReasonInfo disconnectReason);
+
+    /**
+     * Telecom calls this method when a {@link CallQuality} report is received from the telephony
+     * stack for a call.
+     * @param callQuality The call quality report for this call.
+     */
+    public abstract void onCallQualityReceived(@NonNull CallQuality callQuality);
+
+     /**
+      * Signals the active default dialer app to display a call diagnostic message.  This can be
+      * used to report problems encountered during the span of a call.
+      * <p>
+      * The {@link CallDiagnosticService} provides a unique client-specific identifier used to
+      * identify the specific diagnostic message type.
+      * <p>
+      * The {@link CallDiagnosticService} should call {@link #clearDiagnosticMessage(int)} when the
+      * diagnostic condition has cleared.
+      * @param messageId the unique message identifier.
+      * @param message a human-readable, localized message to be shown to the user indicating a
+      *                call issue which has occurred, along with potential mitigating actions.
+     */
+    public final void displayDiagnosticMessage(int messageId, @NonNull
+            CharSequence message) {
+        if (mListener != null) {
+            mListener.onDisplayDiagnosticMessage(this, messageId, message);
+        }
+    }
+
+    /**
+     * Signals to the active default dialer that the diagnostic message previously signalled using
+     * {@link #displayDiagnosticMessage(int, CharSequence)} with the specified messageId is no
+     * longer applicable (e.g. service has improved, for example.
+     * @param messageId the message identifier for a message previously shown via
+     *                  {@link #displayDiagnosticMessage(int, CharSequence)}.
+     */
+    public final void clearDiagnosticMessage(int messageId) {
+        if (mListener != null) {
+            mListener.onClearDiagnosticMessage(this, messageId);
+        }
+    }
+
+    /**
+     * Called by the {@link CallDiagnosticService} to update the call details for this
+     * {@link DiagnosticCall} based on an update received from Telecom.
+     * @param newDetails the new call details.
+     * @hide
+     */
+    public void handleCallUpdated(@NonNull Call.Details newDetails) {
+        mCallDetails = newDetails;
+        onCallDetailsChanged(newDetails);
+    }
+}
diff --git a/telecomm/java/android/telecom/Log.java b/telecomm/java/android/telecom/Log.java
index 2a4fdcb1475df..922eddb6ac3e2 100644
--- a/telecomm/java/android/telecom/Log.java
+++ b/telecomm/java/android/telecom/Log.java
@@ -522,7 +522,7 @@ public class Log {
             return "";
         }
         return Arrays.stream(packageName.split("\\."))
-                .map(s -> s.substring(0,1))
+                .map(s -> s.length() == 0 ? "" : s.substring(0, 1))
                 .collect(Collectors.joining(""));
     }
 }
diff --git a/telecomm/java/com/android/internal/telecom/ICallDiagnosticService.aidl b/telecomm/java/com/android/internal/telecom/ICallDiagnosticService.aidl
new file mode 100644
index 0000000000000..65b4d19b3d9b7
--- /dev/null
+++ b/telecomm/java/com/android/internal/telecom/ICallDiagnosticService.aidl
@@ -0,0 +1,37 @@
+/*
+ * 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.internal.telecom;
+
+import android.telecom.BluetoothCallQualityReport;
+import android.telecom.CallAudioState;
+import android.telecom.ParcelableCall;
+import com.android.internal.telecom.ICallDiagnosticServiceAdapter;
+
+/**
+ * Internal remote interface for a call diagnostic service.
+ * @see android.telecom.CallDiagnosticService
+ * @hide
+ */
+oneway interface ICallDiagnosticService {
+    void setAdapter(in ICallDiagnosticServiceAdapter adapter);
+    void initializeDiagnosticCall(in ParcelableCall call);
+    void updateCall(in ParcelableCall call);
+    void updateCallAudioState(in CallAudioState callAudioState);
+    void removeDiagnosticCall(in String callId);
+    void receiveDeviceToDeviceMessage(in String callId, int message, int value);
+    void receiveBluetoothCallQualityReport(in BluetoothCallQualityReport qualityReport);
+}
diff --git a/telecomm/java/com/android/internal/telecom/ICallDiagnosticServiceAdapter.aidl b/telecomm/java/com/android/internal/telecom/ICallDiagnosticServiceAdapter.aidl
new file mode 100644
index 0000000000000..92eec2a95430d
--- /dev/null
+++ b/telecomm/java/com/android/internal/telecom/ICallDiagnosticServiceAdapter.aidl
@@ -0,0 +1,32 @@
+/*
+ * 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.internal.telecom;
+
+import android.telecom.CallAudioState;
+import android.telecom.ParcelableCall;
+
+/**
+ * Remote interface for messages from the CallDiagnosticService to the platform.
+ * @see android.telecom.CallDiagnosticService
+ * @hide
+ */
+oneway interface ICallDiagnosticServiceAdapter {
+    void displayDiagnosticMessage(in String callId, int messageId, in CharSequence message);
+    void clearDiagnosticMessage(in String callId, int messageId);
+    void sendDeviceToDeviceMessage(in String callId, int message, int value);
+    void overrideDisconnectMessage(in String callId, in CharSequence message);
+}
diff --git a/telecomm/java/com/android/internal/telecom/ITelecomService.aidl b/telecomm/java/com/android/internal/telecom/ITelecomService.aidl
index 5afac2ec239a3..0d190d89645a7 100644
--- a/telecomm/java/com/android/internal/telecom/ITelecomService.aidl
+++ b/telecomm/java/com/android/internal/telecom/ITelecomService.aidl
@@ -351,4 +351,8 @@ interface ITelecomService {
      */
     void setTestDefaultDialer(in String packageName);
 
+    /**
+     * @see TelecomServiceImpl#setTestCallDiagnosticService
+     */
+    void setTestCallDiagnosticService(in String packageName);
 }