In iOS 13 or later, Apple introduces changes in Push Notification handling. If your application supports incoming calls via Push Notification on iOS devices, your application is impacted due to these changes. The new policy mandates that Apps built using Xcode 11 and running on iOS 13 devices, must report all PushKit push notifications to CallKit in the same run loop. Failure to do so will result in iOS 13 terminating your app and the system will stop delivering any more push notifications to your app.
Check our migration guide to upgrade your applications for iOS 13.