unable to communicate when using callkeep & agora in flutter

I've implemented voip call in my flutter app. If both the user open the app and join(agora channel) manually, it's working fine. But, If user A calls to user B, I'm getting call in user B's mobile using callkeep & fcm data message. i.e., in FCM onBackground Message i've written code to enable audio & join channel. Both the lines are executed(channel joined, audio enabled). But, none of the users can speak or listen to other users.

Here's my code.

Future<void> bgMsgHandler(RemoteMessage message) async {
 
  Map msgData = message.data;

  if (msgData['action'] == "Incoming call") {
    final callUUID = "call_uuid_1";

    // #region answer_call_action
    _callKeep.on(CallKeepPerformAnswerCallAction(), (event) async {
      print(
          'backgroundMessage: CallKeepPerformAnswerCallAction ${event.callUUID}');
      Timer(const Duration(seconds: 1), () {
        _callKeep.setCurrentCallActive(callUUID);
        _callKeep.setMutedCall(callUUID, true);
      });
      await initAgora();
    });
    // #endregion

    // #region end_call_action
    _callKeep.on(CallKeepPerformEndCallAction(),
        (CallKeepPerformEndCallAction event) {
      print(
          'backgroundMessage: CallKeepPerformEndCallAction ${event.callUUID}');
      engine.leaveChannel();
    });
    // #endregion

    if (!callKeepInited) {
      _callKeep.setup(
          null,
          <String, dynamic>{
            'ios': {
              'appName': 'CallKeepDemo',
            },
            'android': {
              'alertTitle': 'Phone Permissions required',
              'alertDescription': 'Allow access to speak to our partners',
              'cancelButton': 'Cancel',
              'okButton': 'Ok',
              'foregroundService': {
                'channelId': 'com.company.my',
                'channelName': 'Foreground service for my app',
                'notificationTitle': 'My app is running on background',
                'notificationIcon':
                    'Path to the resource icon of the notification',
              },
            },
          },
          backgroundMode: true);
      callKeepInited = true;
    }
    print('backgroundMessage: displayIncomingCall (user B)');
    _callKeep.displayIncomingCall(callUUID, 'user_B');
    _callKeep.backToForeground();
  }
}

In debug console I am getting message the local & remote users joined. But, unable to transmit & receive audio.

Here's my init agora function

Future<void> initAgora() async {
  engine = await RtcEngine.create(AGORA_APP_ID);

  engine.setEventHandler(RtcEngineEventHandler(
    joinChannelSuccess: (channel, uid, elapsed) {
      print("local user [$uid] joined");
    },
    userJoined: (uid, elapsed) {
      print("remote user [$uid] joined");
      remoteUid = uid;
    },
    userOffline: (uid, reason) {
      print("remote user [$uid] left channel $reason");
      remoteUid = null;
      engine.leaveChannel();
    },
  ));

  try {
    await engine.enableAudio();
    await engine.joinChannel(AGORA_TOKEN, "firstChannel", null, 0);
  } catch (e) {
    print("error with agora = $e");
  }
}

I don't know if it's problem with agora or callKeep. Can you help me to solve this? Any help is appreciatable.