If my application records some audio with MediaRecorder, is it possible to play the recorded audio file into a call that was started with ACTION_CALL or ACTION_DIAL?
This not yet possible on Android. See this feature request.
You can record your voice from microphone, but you can not record the sound of the other party.
android.intent.action.ANSWER may be what you are looking for, or possibly PhoneStateIntentReceiver.