Nokia Lumia 800 - Why does my Nokia Lumia's screen turn black when in a call?

The screen gets switched 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. To switch the screen on, press the power (lock) key.

If the screen gets switched off immediately when dialling a number or receiving a call, check that the proximity sensor on the top of the screen is not covered by any kind of protective film or tape.

You can take the following steps: If the screen remains switched off, the protective film may not be of the right size and will block the sensor on the upper part of the screen. Either remove the protective film or cut off the piece of film that is blocking the area. Then, check the issue as follows:

  1. Make or receive a call with the phone.
  2. Once the phone is in a call, place your hand 1cm above the screen and the display will switch off.
  3. Remove your hand and the display will switch back on.
  4. Repeat steps 1, 2, and 3 three times to verify the proximity sensor function.