Recently, after owning the Lumia 920 for about 3 months, I noticed that I can’t turn the screen on while taking call. During that time I’ve updated to Portico & dust started accumulating in the front facing camera (FFC). So I googled the problem & came up with this thread at Nokia Forum.
This is a problem since launch & appears to be related to dust in the FFC lens. It’s a known problem that the FFC lens housing is not completely sealed.
The solution for me is to use the front camera a few times with different light source, just like how you need to calibrate the front camera to get the correct screen brightness adjustment, you need to calibrate it for calling as well. I’ve tried to get rid of the dust in the FFC lens by vacuuming, didn’t work. Will try a lens blower next if not, I’d just have to open up my Lumia 920.
Apparently either the proximity OR the FFC will switch off the screen. Why not just use the proximity sensor only? My Lumia 800 had no issues when I place the phone to my face when calling. Unless Nokia telling me the screen wouldn’t switch off in the desert…
Anyway the situation seems to be better now that I “calibrated” the FFC.
I got the 1308 update almost 2 weeks ago which completely solved the problem for most people. As far as I can tell, when the dust covers about 30-40% of the lens, the “calibration” technique will fail because not enough light falls onto the sensor. The 1308 seems to switch on the FFC longer to get a more accurate “averaged” reading thus solving the dust blockage issue.
Of course, the ultimate solution is to send the phone in for cleaning & replacement of the FFC casket.