Update documentation to GetPhoneNumber with the security/privacy concerns

Change-Id: I00b863d4aff9354ea486bcecd8b67f33e9ee7972

telephony/java/android/telephony/SubscriptionManager.java

@@ -3952,6 +3952,10 @@ public class SubscriptionManager {
      * may provide one. Or, a carrier may decide to provide the phone number via source
      * {@link #PHONE_NUMBER_SOURCE_CARRIER carrier} if neither source UICC nor IMS is available.
      *
+     * <p>The availability and correctness of the phone number depends on the underlying source
+     * and the network etc. Additional verification is needed to use this number for
+     * security-related or other sensitive scenarios.
+     *
      * @param subscriptionId the subscription ID, or {@link #DEFAULT_SUBSCRIPTION_ID}
      *                       for the default one.
      * @param source the source of the phone number, one of the PHONE_NUMBER_SOURCE_* constants.