Well apparently several things including third party apps or screen cover could cause this issue of ‘having the proximity switch constantly ON’. However in my case (I have Xperia Z Ultra with Lollipop) the issue is hardware related. As many already suggested here, the proximity switch’s chip is located somewhere close to the proximity sensor inside the phone. If you happen to put enough pressure on that area then your proximity switch becomes off and it works fine afterward while you are still applying that pressure. Apparently those who are lucky, by keep having that pressure for few secs the switch’s chip slips back into its position and the problem would be resolved. However for me, unfortunately, as soon as I remove the pressure the switch turns back on! You can check that by dialing *#*#7378423#*#* and then in the Service Menu go to the ‘Service Test’ and hit the ‘Proximity Switch’. If you have the same problem your proximity switch should appear as ‘ON’ there. When you apply that pressure somewhere around the sensor ( mine is on the back close to the NFC logo) you would see on screen that proximity switch is going OFF. Hopefully after you remove the pressure it stays off and you would solve the problem this way, but if you are not lucky like me then you could either do the hardware troubleshooting by literally having access to the proximity chip or try the dummy thing that I do (but works every time) , which is pressing that same area on the back of my phone when I want to hang-up and then bring the phone down from my ear and disconnect the call or have access to screen while I am still on the line!
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