During a call, if the phone isn't being held to your ear, the screen will go off after 10 seconds. Pressing the power button will wake the screen up again. If you tap the screen every few seconds while on call, the screen will stay on. Each tap on the screen will reset the 10s timer.
Quite an annoying feature tbh. It's needs an option to keep the screen on if you want to, without doing the above.