In iOS 14 you can now accept phone calls that do not take up the entire screen (the new non-intrusive phone call UI looks like a push notification). If you accept the call, but do not open up the full screen call UI, the app remains open. This is true even if you pick up the phone and put it to your ear the app remains active.
What would the best way be to detect an incoming call, or that the user began the call?