I have less or no experience in developing calling apps. In my android app I have copied all content from your sample call app & the app functioning is OK as in your sample app.
Some call app functions are missing & need to add them:
No dial tone heard when making a call.
As of now only a notification is shown when an incoming call is arrives. Need to wake up the screen if locked show an activity with Caller’s name/username & accept & reject buttons (or show high priority notification for Android 10+ with accept & reject button when screen is awake) & play device ringtone (depending on device silent settings). Accept should go to VoiceCallActivity or VideoCallActivity & Reject should close the activity or dismiss notification
If anyone has added these functions please share examples or sample code.