What's the best way to notify offline user about incoming call

Hello everyone, I was wondering what’s the best way to let offline user know that someone is calling them.

Let’s imagine we have user A and user B. User B is offline, the app is killed, user A tries to call user B.

What’s the best way to notify user B about it? I can only think of push notification but it’s very easy to miss it, is there a way to implement call behavior or maybe you guys have better ideas?

P.S. I’m using socketio to connect users
P. P.S. I’m looking for something that will work both on iOS and android

Android notifications can be a little bit of a pain due to many factors but i’d say go the notification route with simple data notifications that call back to the backend for confirmation before launching the app call screen.
iOS is a little less of a pain due to having VOIP notifications specifically for this task which tends to be more reliable.

You could also go down the route of having a background service connecting to your backend and wait for responses on and off.
Issue with that is extra battery/network usage and theres still a lot of factors which would cause problems.
Also i’m unsure if you can do that on iOS as easily, then again on both sides you could do scheduled fetching but that isn’t a nice thing really.

Might wanna check this.