I am using Linphone lib on iOS (and Flexisip on server) to build VoIP app on iOS.
Everything working well. Only when mobile app in background mode and have incoming call, my app SOMETIME not working (not wakeup and receive incoming call).
When incoming call, Flexisip server will request to APNS to push notification to destination mobile app. My mobile app will use PushKit to receive and process this notification (as show incoming call screen)
However, sometimes it doesn't working.