I’m trying to implement custom call screen when app receive certain push notification.
When I have all the data avalable, I’m trying to raise local notification while app is still in background.

NB: it’s a .NET code for MAUI, but it’s basically a complete copycat of Android API, so should be readable for Java programmers.

void RegisterNotification(string cid, string title){
.......
var clickIntent = new Intent(context, typeof(MainActivity));
clickIntent.AddFlags(ActivityFlags.SingleTop | ActivityFlags.ClearTop);
clickIntent.PutExtra("field1thatneeded", "abcde");

var pendingIntentFlags = (Build.VERSION.SdkInt >= BuildVersionCodes.S) ? PendingIntentFlags.UpdateCurrent | PendingIntentFlags.Immutable : PendingIntentFlags.UpdateCurrent;
var pendingIntent = PendingIntent.GetActivity(context, activityId, clickIntent, pendingIntentFlags);

.......

var notificationBuilder = new Notification.Builder(this, AppConstants.CallChannelID)
   .SetAutoCancel(false)
   .SetOngoing(true)
   .SetContentTitle("Incoming call")
   .SetContentText(title)
   .SetContentIntent(pendingIntent)
   .SetStyle(Notification.CallStyle.ForIncomingCall(incomingCaller, rejectPendingIntent, answerPendingIntent))
   .SetCategory(Notification.CategoryCall);

var notification =notificationBuilder.Build();

#if ANDROID29_0_OR_GREATER
            StartForeground(notificationID, notification, Android.Content.PM.ForegroundService.TypePhoneCall);
#else
            StartForeground(notificationID, notification    );
#endif
}

Creating call notification right away not possible before any activity is attached, I cannot get PendingIntent (Attempt to invoke virtual method ‘android.os.UserHandle android.content.Context.getUser()’ on a null object reference) or feed builder with required resources (Attempt to invoke virtual method ‘android.content.res.Resources android.content.Context.getResources()’ on a null object reference / Java.Lang.NullPointerException: Attempt to invoke virtual method ‘android.os.UserHandle android.content.Context.getUser()’ on a null object reference )

The only solution I found is to create a Service:

[Service(ForegroundServiceType = Android.Content.PM.ForegroundService.TypePhoneCall, Exported = false)] //this attribute fills up AndroidManifest

internal class DroidCallService: Service
{
public override StartCommandResult OnStartCommand(Intent intent, [GeneratedEnum] StartCommandFlags flags, int startId)
{
    if (intent.Action == "START_SERVICE")
    {
        var callID = intent.GetStringExtra("call_id");
        var name = intent.GetStringExtra("call_title");
        RegisterNotification(callID, name);
    }
    else if (intent.Action == "STOP_SERVICE")
    {
        StopForeground(StopForegroundFlags.Remove);
        StopSelfResult(startId);
    }

    return StartCommandResult.NotSticky;
}
}

When app is unloaded, all this work fine on Androids 11 and older, but since new restrictions of foreground services run, I get error on starting a service:

            Intent startService = new Intent(ctx, typeof(DroidCallService));
            startService.SetAction("START_SERVICE");

            startService.PutExtra("call_id", callID);
            startService.PutExtra("call_title", name);
            
#if ANDROID26_0_OR_GREATER
            ctx.StartForegroundService(startService); // ERROR!
#else
            ctx.StartService(startService);
#endif

Android.App.ForegroundServiceStartNotAllowedException:
startForegroundService() not allowed due to mAllowStartForeground
false

and

Android.App.BackgroundServiceStartNotAllowedException: Not allowed to
start service Intent { act=START_SERVICE
cmp=com.company.app/crc646332b1bcddc0e22c.DroidCallService
(has extras) }: app is in background

I saw multiple recommendations like asking user to skip battery optimization, using alarm clock or calendar etc, but it’s all like “quirky” solutions.

Task is simple: background instance needs to run a foreground service. What is the proper way to do it and how?