I owned one of these temporarily (long story). Anyway, the proximity sensor is somewhere near the Sony logo at the top of the phone. Its almost like a camera in its functionality. I found that dirty glass or even a glossy screen protector interfered with it. I suppose if you could take off the top glass, you can replace or clean the sensor.
Here is the tear down guide and see if it is worth the trouble or not.
[guide|52300]