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Big daddy resolved it 
You have to sacrifice resetting the phone using the Sony PC software 1st. Avoid data backup from old accounts.
Then, after the reboot is done and Android is reinstalled with update x.x.x.534 goto the dialer & type *#*#7378423#*#* then press dial
In the menu that will come go to calibration and then choose proximity sensor. (If you run the test it might give error = -1) Then, set settings to default and it may ask to reboot the device.
After reboot type *#*#7378423#*#* then press dial. In the menu that will come go to calibration and then choose proximity sensor do the test and it will show success. VIOLAAAaaaa!!!!
Check the phone...it works 
Have tested this and the issue is resolved on my phone finally ....yippieeeeeee!!!
my sony xperia c does not disable screen during calls. when do the tests, it says "proximity switch not detected".
i shall be grateful for your advise.
buddyyyy worked!!!!
I do not find can not find Calibration menu after kit-kat update, as there was i can fix it?
I bought. Xperia z ultra 2 days back.
I updated it to latest version 4.4
I m from india.
Proximity sensor is not working
Caliberation option not available when i tap *#*#7378423#*#*
Is there aany other way
My email is please advice.
upgrade to jellybean or later