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 the proximity sensor area at the upper front of the screen is not covered by any kind of protective film or tape.
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.