This isn't my answer but I'm having the same problem and this is what I found
"In case anyone else is banging their head against a wall with this. I will provide my solution. It seems that voip notifications need to report to call kit within seconds so it has to be done right away. I was using an API call to get the call details first and then reporting the call. This will cause an error to show up that says the same thing I just explained. If you keep sending notifications without reporting right away after about 4 times Apple will block the notifications from coming in. From your notification server you will see 200 success but nothing on the app side. I had to delete the app and install again to see the error message again, then reporting the call to CallKit right after seems to work. Background and killed state. I did use a serial queue (not main). Good luck to anyone else out there facing this issue as the threads are bare bones on answers. Hope this helps."