Frankly, the proximity sensor should close the screen when it is to your ear so that doesn't happen. You can check the functionality of the proximity sensor by dialing your voicemail and, while connected, place your palm over the front camera. The screen should go off. The only way it comes back on is when I move my hand away from the camera cutout. You may be unconsciously moving the phone away from your ear while keeping the lower part pressed to your cheek. As soon as the camera senses light, the screen goes back on. The proximity sensor is there for the express purpose of preventing accidental button pushes by one's face when the phone is in use. Many people do tend to hold the phone away from their ear, especially if they have the volume up, as it can be uncomfortable if one is wearing earrings, has sensitive ears, or is on the phone too long. So, a couple of workarounds I can suggest would be 1) make sure you have the phone flush up against your ear, 2) double tap the screen to lock it when you connect to a phone call or, 3) return to your home screen after the call is dialed or picked up. The call will stay active, but the dial pad will be hidden.
