To stop people from inadvertently interacting with the iPhone's screen while they're on a phone call, the iPhone comes equipped with a built-in proximity sensor which automatically turns off the screen when it detects that an opaque surface is in front of it, like the user's cheek. The sensor becomes activated once the user begins to make a phone call.
Open the device's native "Phone" application and then tap the dialer within the application to reveal the keypad.
Dial the phone number that you'll be using to make the test call, then begin the call.
Place the phone to your ear as you normally would or block the upper portion of your iPhone with your hand or another opaque surface.
Take note that the screen will activate and light up once the proximity sensor is no longer blocked by your ear or other surface.