Можно использовать PhoneStateListener, чтобы начать предыдущую активность после завершения вызова.
class CallEndedListener extends PhoneStateListener {
boolean called = false;
@Override
public void onCallStateChanged(int state, String incomingNumber) {
super.onCallStateChanged(state, incomingNumber);
if (state == TelephonyManager.CALL_STATE_OFFHOOK)
called = true;
if (called && state == TelephonyManager.CALL_STATE_IDLE) {
called = false;
// do something here.
}
}
}