Is it possible that you cover the proximity sensor with your hand or something else while holding the phone, because the screen goes blank when you make a call only when you put it to your ear which covers the proximity sensor next to the earpiece and it makes sure you don't accidentally click anything while on call.
Just make sure your sensor is working properly.
Other than that i'd suggest you enable the 'hang call by pressing the lock button' option, that way you can end the call with the lock button and without waking up the screen. Its not a solution to the problem but should help until you find one.
