Broadcast intent action indicating that the call state (cellular) on the device has changed.
The TelephonyManager.ExtraState extra indicates the new call state. If the new state is RINGING, a second extra TelephonyManager.ExtraIncomingNumber provides the incoming phone number as a String.
Requires the READ_PHONE_STATE permission.
This was a Android.Content.Context.SendStickyBroadcast(Android.Content.Intent) broadcast in version 1.0, but it is no longer sticky. Instead, use TelephonyManager.CallState to synchronously query the current call state.