diff --git a/telephony/java/android/telephony/CarrierConfigManager.java b/telephony/java/android/telephony/CarrierConfigManager.java
index df8affef8f464..8889ca9f0a34e 100644
--- a/telephony/java/android/telephony/CarrierConfigManager.java
+++ b/telephony/java/android/telephony/CarrierConfigManager.java
@@ -266,6 +266,16 @@ public class CarrierConfigManager {
     public static final String KEY_CARRIER_INSTANT_LETTERING_AVAILABLE_BOOL =
             "carrier_instant_lettering_available_bool";
 
+    /**
+     * When IMS instant lettering is available for a carrier (see
+     * {@link #KEY_CARRIER_INSTANT_LETTERING_AVAILABLE_BOOL}), determines the list of characters
+     * which may not be contained in messages.  Should be specified as a regular expression suitable
+     * for use with {@link String#matches(String)}.
+     * @hide
+     */
+    public static final String KEY_CARRIER_INSTANT_LETTERING_INVALID_CHARS_STRING =
+            "carrier_instant_lettering_invalid_chars_string";
+
     /**
      * If Voice Radio Technology is RIL_RADIO_TECHNOLOGY_LTE:14 or RIL_RADIO_TECHNOLOGY_UNKNOWN:0
      * this is the value that should be used instead. A configuration value of
@@ -432,6 +442,7 @@ public class CarrierConfigManager {
         sDefaults.putBoolean(KEY_CARRIER_VOLTE_TTY_SUPPORTED_BOOL, true);
         sDefaults.putBoolean(KEY_CARRIER_ALLOW_TURNOFF_IMS_BOOL, true);
         sDefaults.putBoolean(KEY_CARRIER_INSTANT_LETTERING_AVAILABLE_BOOL, false);
+        sDefaults.putString(KEY_CARRIER_INSTANT_LETTERING_INVALID_CHARS_STRING, "");
         sDefaults.putBoolean(KEY_DISABLE_CDMA_ACTIVATION_CODE_BOOL, false);
         sDefaults.putBoolean(KEY_DTMF_TYPE_ENABLED_BOOL, false);
         sDefaults.putBoolean(KEY_ENABLE_DIALER_KEY_VIBRATION_BOOL, true);