For call UI you need to do following setup, so the call feature can work properly
In your project target, select signing & capabilities and add Background capabilities.
Now, go to the background mode and enable the below given modes
|Audio,Airplay, and Picture in Picture|
|Voice over IP|
Add Microphone privacy description in your project plist
Add Camera privacy description in your project plist
Add Caller ringtone & reconnecting tone mp3 file in your project.
The file name must follow in the following manner
|.mp3 file||File Name|