The phone screen was locked when I answered a call. After the person on the end of the call hung up, I looked at my screen and the top 1/3 area was in a different color and said "In full screen mode." So I swiped up to get rid of it then the red phone circle appeared and the phone connection time started counting 0:01, 0:02,... I said hi, and the same person on the other end said "I don't know my phone just started ringing. So I pressed the red phone circle to hang up and again the red phone circle appeared and the time restarted counting up again. I hung up again and it finally stayed hung up. If I hadn't noticed, it would be like the connection stayed open for someone to remotely listen.
I don't know what "Full screen mode" means while the display is locked. I don't know how it kept reestablishing the call after I kept hanging up.