code: public void setCallState(int callState, String incomingPhoneNumber) {
      if (callState != CALL_STATE_RINGING) {
        incomingPhoneNumber = null;
      }
  
      this.callState = callState;
      this.incomingPhoneNumber = incomingPhoneNumber;
  
      for (PhoneStateListener listener : getListenersForFlags(LISTEN_CALL_STATE)) {
        listener.onCallStateChanged(callState, incomingPhoneNumber);
      }
    }
docs: Sets the current call state with the option to specify an incoming phone number for the
  CALL_STATE_RINGING state. The incoming phone number will be ignored for all other cases.