Where is the proximity sensor that everyone refers to when the screen goes black during calls?
It is usually up next to the speaker that you put to your ear when on a call. The reason for this is that is supposed to detect that it on your er (no light going in) to turn the screen off while on a call. It also is what controls the screen brightness when you have Auto Brightness enabled.
The sensor is located differently on each phone model. Which phone model do you have?