I recently got my phone fixed because the digitizer broke and when I got it back I updated my phone to kit Kat now when I make calls the screen goes immediately black or turns off I can still talk and hear the other person but I can't access my screen during the call, every time I press the power button the screen just flashes I'm not sure if its because of the update or the People that fixed my phone messed up
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Sounds like the proximity sensor may be having trouble detecting whether or not the phone is being held up to your ear. Some cases can interfere or cover the sensor (the little round spot at the top-left of the front glass); otherwise, I'd go back to whoever fixed the phone and get them to verify that the proximity sensor wasn't damaged in the process.