The display turning off is a feature that prevents the device from registering touch inputs when you press your face against the phone during a call. It?s triggered via the proximity sensor on the top left corner of the phone. If take the phone away from your face (i.e., uncover the proximity sensor), the display should turn back on automatically.
Anyway, you should go to ?Settings\Accessibility? and toggle off ?Power button ends call?. This will allow you to manually switch on the screen without ending your call.