Hah the nightmare to get CallKit working. The only thing you could do is take a working example application and step by step wrap your application around it.
Because taking your existing VoIP application and inserting CallKit behavior breaks in so many mysterious and undocumented ways that I really had to tell a client it wasn't going to happen with his existing application within the allocated budget.
Just to get the fancy Apple call screen!
And of course only FaceTime can take video calls straight into video from the lock screen, so you have to explain after it works why nobody can see each other.
Because taking your existing VoIP application and inserting CallKit behavior breaks in so many mysterious and undocumented ways that I really had to tell a client it wasn't going to happen with his existing application within the allocated budget.
Just to get the fancy Apple call screen!
And of course only FaceTime can take video calls straight into video from the lock screen, so you have to explain after it works why nobody can see each other.