This one:
https://developer.apple.com/videos/play/wwdc2016/230/
I can summarise it like this: VoIP calls will be visually and functionality indistinguishable from regular calls and FaceTime calls. They use the same UI and adopt most of the same behaviour. When you receive an actual call while you are VoIPing using this API, then you can deal with it in the same way: switch to the call or put in on hold. They are also integrated in the phone app and contacts, Siri, Bluetooth and the do-not-disturb mode.