Linphone iOS Push notifications

I am running RasPBX server.

I have two iPhones running Linphone, each one is registered to the RasPBX server.

Everything works fine over my local network when the Linphone is not in the background.

If I leave the app or the iPhone is on the lock screen, I do not get alerts for incoming calls.

Can anyone tell me what I need to do receive push notifications about incoming calls?

This would be a question for Linphone. There is no standard way to do push notification, and it requires a running server, as well as certificates with the respective mobile OS provider (Apple or Google).