Hello. That happened to me yesterday when I was using someone's Sony Z3 to phone support in a carrier store. Lady I was speaking to said I must have muted it. I guess if you are moving the phone away from your ear and back while you speak, your ear can hit the mute switch before the proximity sensor turns the screen off again. or you may have a faulty sensor that doesn't turn off the screen.
Try lying it on a desk while making a call and swiping a hand over the sensor near the top which should turn the screen black.