Why is screen turning black when trying to phone?

[AC Question]

When you try to phone someone the screen turn black and won't turn on again. Had to take the battery out every time to get it disconnected because you can't see what's happening with the connection. What can you do to get it fixed? Please help!

 

[belodion]

I suppose that the proximity sensor is readied when you open the Phone app. If it does its stuff prematurely, I suppose it could be because of a case, because the sensor is faulty, or because of some software problem. Are you using a case?