How to know Phone State as a service?

The Problem:

What I want the app to do is checking a phone's current call state in the background. Now, for making an app that works in background we need a service(source codes in the next section), but the only way I have managed to do this is by using a receiver. The issue with the receiver is that it is not always running if you kill the app in the background. Now this seems a fairly simple question, but I am new to JAVA.

Source Codes

This is my main service's OnStartCommand file:

@Override
public int onStartCommand(Intent intent, int flags, int startId) {
Notification notification = new NotificationCompat.Builder(this, CHANNEL_ID).setContentTitle("asdaff").setContentText("asdas").setSmallIcon(R.drawable.ic_android_black_24dp).build();
startForeground(1, notification);
TelephonyManager telephonyManager =
(TelephonyManager)getSystemService(Context.TELEPHONY_SERVICE);
PhoneStateListener callStateListener;
callStateListener = new PhoneStateListener() {
public void onCallStateChanged(int state, String incomingNumber) {
String stateString = "N/A";
switch (state) {
case TelephonyManager.CALL_STATE_IDLE:
stateString = "Idle";
Toast toast = Toast.makeText(getApplicationContext(), stateString, Toast.LENGTH_LONG);
toast.setGravity(Gravity.CENTER, 0,0);
toast.show();
break;
case TelephonyManager.CALL_STATE_OFFHOOK:
stateString = "Off Hook";
Toast toast1 = Toast.makeText(getApplicationContext(), stateString, Toast.LENGTH_LONG);
toast1.setGravity(Gravity.CENTER, 0,0);
toast1.show();
break;
case TelephonyManager.CALL_STATE_RINGING:
stateString = "Ringing";
Toast toast2 = Toast.makeText(getApplicationContext(), stateString, Toast.LENGTH_LONG);
toast2.setGravity(Gravity.CENTER, 0,0);
toast2.show();
break;
}
}
};
return super.onStartCommand(intent, flags, startId);
}

Edit

Used startForegroundservice() and it still does not work. I have the phone permission. Please tell if you need more of the source code to tell the problem.

So I figured it out. The issue is in the onStartCommand method itself. When using a phone state listener, you need to have the listener registered to the telephony manager.

telephonyManager.listen(callStateListener, PhoneStateListener.LISTEN_CALL_STATE);

Now, I thought this is one of those "Google-able" Things, that have a proper beginner mistake warning that may help anyone who is going to commit such a stupid mistake(i.e, me).

I am Writing this answer so that anyone who comes across this may no what not to do.
Cheers.