Marty has a Samsung Note 4 that goes to a blank screen when he's on calls. Then he has to touch the power button to turn it back on. Leo says that there is a proximity sensor that turns off the screen when you put it up to your ear. That way your cheek doesn't hit any buttons on the screen. So it sounds like that sensor is a bit out of wack.
Marty could try resetting his phone, but if that doesn't fix it, it would be a hardware issue and that means either getting a new phone, repairing it, or living with it.