hi there,
i basically have the same problem.
as suggested in the tutorial by fairphone i checked the proximity sensor with the "sensors" app. and it turned out that it does not work (the app shows 0.000 cm no matter if there is something close to the sensor or not).
so i thought it's a matter of pollution as described in the other answers. my cleaning efforts didn't help. then i realized: when i call my mailbox to check messages it works very well! but obviously it is another sensor doing the work here: the screen turns black NOT if i hold it close to my head but if i hold it upright (which you mostly do when you hold it to your ear). and the screen works again properly as soon as i put the phone down into a horizontal position. so it seems as if the proximity sensor does it's work, actually it doesn't. and: this doesn't work with any other calls.
did anyone observe something similar?!
best wishes,
rainer.