I have noticed that while on a call and when trying to check my notifications or whatever, that the screen shuts off because of the proximity sensor. When I use my left hand and try to do this it usually shuts the screen off. Don't know if that helps you or not.
What I sometimes do is use the Home button to go to the home screen, the call is still active but the screen only shuts off when the phone app is in the foreground. Then you return to your call screen through the notification bar again.