I'm currently developing a voip for a client. We've successfully got the app working using Xcode 11 and SwiftUI. So far all the push notification work has gone smoothly. We can reliably start and end calls using the voip push notifications.
A problem has come up though. If a user is already in a GSM call then at the moment a Voip push notification arrives they see an alert pop up that says "call failed" and their call ends. In our debugging we do not see our own app wake up and receive the voip push.
If we stop using voip notifications and just use regular push notifications then this problem goes away. There seems to be some difference in how ios13 handles the voip push notifications that causes it to break existing GSM calls.
Interestingly if our app is already in a call and a voip push arrives this problem does not occur and we can handle the voip push as normal.
For now we are going to deploy the app with support for both voip and regular push notifications and our server infrastructure will use the regular notifications till we have a fix for this.
