I am using GrapheneOS with Google services installed (GrapheneOS is Android 16 AOSP with additional security enhancements) and Zoiper5 does not always ring. However, I can see incoming calls on the screen, just the ringer does not work.
I disabled battery optimisation for Zoiper and also allowed all notifications. I can normally call, and if I see the incoming call, I can answer.
Now the weird part. I am using RasPBX and internal calls (from local extensions ring normally). But when I receive a call from the outside trunk, it does not ring. In that case it seems like ringer is muted (but it is not).
Any idea what could be wrong or where to start looking for debugging?