There is a sensor that shuts off the screen during calls if it is covered. It is left of the speaker on the front. Make sure it is not being covered

 

You can see the notch on my screen protector just left of the VM logo, that is the proximity sensor.

 

Yeah, it's just so when you have it pressed to your ear, you won't accidentally end the call or open an app or anything. If you use some headphones or earbuds with it and hold it in your hands, you can still talk and multi-task on your phone at the same time if you like.