Full screen notification like in-coming call in android call app

Question

I am working on video call android app.My challenge now is to implement full screen notification similar to android built in Call app when you have in-coming call. Currently Whenever i get a call the notification appears as regular notification if the phone screen is off and heads up if the phone is awake.I want the notification to occupy fullscreen just like the call app.I am using Galaxy S5 to test it.What am i missing here is my code.Thank you in advance.

        Intent intent = new Intent(this, MainActivity.class);
        intent.putExtra("screen", "debugScreen");
        intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK
                | Intent.FLAG_ACTIVITY_EXCLUDE_FROM_RECENTS
                | Intent.FLAG_ACTIVITY_NO_USER_ACTION);
        Intent callAcceptIntent = new Intent(this, MyReceiver.class);
        callAcceptIntent.setAction("com.videochat.CALL_INTENT");

        PendingIntent callAcceptPi = PendingIntent.getBroadcast(this,0,callAcceptIntent,0);
    PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, intent, 0);

        NotificationCompat.Builder mBuilder =
            new NotificationCompat.Builder(this)
                    .setContentIntent(pendingIntent)
                    .setSmallIcon(R.drawable.icon)
                    .setContentTitle("My notification")
                    .setContentText("Hello World!")
                    .setTicker("ticker")
                    .setFullScreenIntent(pendingIntent, true)
                    .setOngoing(true)
                    .setCategory(NotificationCompat.CATEGORY_CALL)
                    .setVisibility(NotificationCompat.VISIBILITY_PUBLIC)
                    .setPriority(NotificationCompat.PRIORITY_MAX)
                    .addAction(R.drawable.icon, "Reject",callAcceptPi);

                NotificationManager mNotificationManager =
            (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);