The screen gets turned off during a phone call when the proximity sensor is covered. This is done to avoid accidental touch screen actions when you are in a call.
If the screen gets turned off immediately when dialing a number or receiving a call, check that your phone does not have any object which may cause detection. There are various non-Nokia original screen protectors sold in the market which cover the proximity sensor causing blank screen issues during calls.
You can take the following steps to check the issue. If the screen keeps turned off, the protector may not be of the right size and will block the sensor on the upper part of the front side. Then either remove the protector or cut off the plastic from the blocking area.