Just FYI, you can set those notifications to be ‘silent’ (not displayed on the top) and to collapse into a thin line each—in the system's notification settings. The apps still keep running, however I haven't figured out whether this change affects the frequency with which the background service is called by the system, and thus synchronization delays.

Don't forget that turning off the screen can wildly affect service behavior even for foreground services, and these effects vary across vendors:

https://developer.android.com/training/monitoring-device-sta...

Linphone does show a persistent notification while running and registered to a server.

On iOS that's pretty much all been removed. The access gateway has to send an APNS notifications when there is an inbound call.

The Android SIP stack is pretty crummy. Last I tried to use it there was no support for TLS registration (leaving your calls unencrypted, barebacking the web) or for push notifications.

Linphone, Zoiper, etc can show a badge in your notifications menu/top bar at all times and get semi-reliable access to run in the background, but expect to miss 5% to 20% of all incoming calls. Firebase push notifications are mandatory if you care about battery life or reliable inbound calling :c

> It's even exposed in the standard Android phone dialler from Google.

If I recall correctly Samsung modifies the standard android dialler to disable this functionality. Or at last they did in the S5 that I used.