Native cellular call fails on VoIP incoming call in iOS 13

This is probably an iOS 13 bug and, if you haven't already done it, you should report it to Apple.

I think that the reason why apps like Whatapp (and the one I develop) are working, is that we build the app against the iOS 12 SDK. We do this because of the limitations of VoIP push notifications introduced in iOS 13. So, you can try to work around the issue—at least until April 2020—building against the iOS 12 SDK. Hopefully, Apple we'll soon fix this issue.