[adihere]

try this <taken from xda-developers>

Lightsensor Switching the Screen off- realised by yelitterulez, confirmed by pietrucci and tested by myself. When making a call the screen stays on if you leave the phone lying flat on a table. If you wait 10 seconds into the call, you can pick it up to your ear and the screen stays on.

BUT if you make a call with the phone slightly tilted/ slanted (you get the idea), the screen turns off as soon as the call gets connected!
The moment you hold the phone to your head the blacklight turns off. In fact: the moment you hide the screen with anything it turns off! (try your hand...).
This might be the reason why everybody's complaining about the screen turning off during a call: you hold the phone to your head! When you turn the screen back on manually, it stays on no matter if you hold your hand or head...