Hello there, testing here Zoiper Pro with push notifications, this seems to be the only architecture that is working finally on mobile phones! No other SIP software works once the mobile phone falls asleep.
Only issue is that after 1 ring the call is dropped.
I am using a testing facility with a test PBX so that only "internal" calls to given extensions are allowed, so these are not outbound calls on real phone numbers. Using classic SIP apps (3CX, GrandStream, etc) would work with the same settings.
Same problem on Android an IOS