When calling my screen goes black on LG P880 After updating in Android 4.1.2 my screen goes black. Screen works again only when the call is finished.
I know about the feature when you put the phone on ear it goes black and the second you remove the phone from your ear , the screen turns on again. 
But now is different. Now it goes black the same second when I press call and there's nothing that brings it back until the call is over!
Please help.
Based from OP's comment, you have to calibrate the proximity sensor:


Go to "Secret Menu" by dialing 3845#*880#
Select "Device Test"
Select "All Auto Test Menu Full"
Choose "ALC / Proximity Test"