Hi,

after I changed screen on my favourite old gal N9, I had same problems,
display turning off after call accepted (no way to interrupt call
other than phone reset), alarm clock couldn't wake up the display...

Thanks to suggestions from guys in this forum:

I tried prox_enable stuff, but without success -> that was pointing to HW problem.
Observed that the file content was restored to "1" after each call.

I transfered the proximity sensor from old screen to the new one,
so I suspected the sensor was misaligned. After some trials I found
sensor was working when screen was not fit back in the case.
Obviously the glue on sensor wiring wasn't strong enough to keep sensor
aligned after pushing screen back to the case.
Fixing wiring in place by tape solved the problem.

Just my five cents...