You can read about that sad story here:
So about 3 days back I went out and bought a new Frame+Display/Digitizier and a new Battery.
Everything was great except for the proximity sensor.
It keeps on turning off the screen during a call because the sensor thinks its covered up.
A bit if googling and I can see that this is a common issue with people who have done a screen change.
The thing is that the fix is pretty simple. All I did was that I took off the back cover and the plastic frame on top of the motherboard and placed a small folded sheet of paper behind the sensor module. I then reinstalled the plastic frame and the glass back. The sensor now works perfectly.
The thing is that I guess the sensor became a little loose and the air gap between the proximity sensor and the front glass resulted in a faulty sensor reading.
The paper presses the sensor module down and eliminates the gap between the sensor and the front glass, thus fixing the problem.
If you dont have a warranty and really want to fix your phone easily, try it.
It took me about 3 minutes to do the whole thing.