How to get telephoney listen after closing the app in flutter ?

In Flutter, you can use the WidgetsBindingObserver and AppLifecycleState classes to listen for changes in the app's lifecycle and perform specific actions, such as listening for phone calls even after the app has been closed.

Here is an example of how you can use these classes to listen for phone calls after the app has been closed:

1. Create a new class that extends WidgetsBindingObserver and implements the didChangeAppLifecycleState method.

class LifecycleEventHandler extends WidgetsBindingObserver {
  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    switch (state) {
      case AppLifecycleState.inactive:
        // Listen for phone calls here
        break;
      case AppLifecycleState.paused:
        // Listen for phone calls here
        break;
      case AppLifecycleState.resumed:
        // Listen for phone calls here
        break;
      case AppLifecycleState.suspending:
        // Listen for phone calls here
        break;
    }
  }
}

2. In your main() function, create an instance of the LifecycleEventHandler class and add it as an observer to the WidgetsBinding

void main() {
  WidgetsFlutterBinding.ensureInitialized();
  final lifecycleEventHandler = LifecycleEventHandler();
  WidgetsBinding.instance.addObserver(lifecycleEventHandler);
  runApp(MyApp());
}

3. Inside the didChangeAppLifecycleState method, you can use the FlutterPhoneState package to listen for phone calls and perform actions when a call is detected.

import 'package:flutter_phone_state/flutter_phone_state.dart';

class LifecycleEventHandler extends WidgetsBindingObserver {
  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    switch (state) {
      case AppLifecycleState.inactive:
      case AppLifecycleState.paused:
        FlutterPhoneState.startPhoneStateListening((PhoneState phoneState) {
          if (phoneState == PhoneState.CALL_OFFHOOK ||
              phoneState == PhoneState.CALL_RINGING) {
            // Perform actions when a call is detected
          }
        });
        break;
      case AppLifecycleState.resumed:
        FlutterPhoneState.stopPhoneStateListening();
        break;
      case AppLifecycleState.suspending:
        FlutterPhoneState.stopPhoneStateListening();
        break;
    }
  }
}

Please note that in iOS, you may need to add the NSMicrophoneUsageDescription key to your app's Info.plist file and set its value to a string describing why your app needs to listen for phone calls.