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What phone do you have? Also an LG Optimus G Pro? Is the screen going blank when you hold the phone up to your ear, and then not turning back on after removing the phone from your ear? That could be a proximity sensor problem--make sure you don't have a dirty screen protector or a case that might be blocking the sensor (which is usually near the front facing camera). When the screen blanks out, can you still talk to the other person on the phone? What about when the other person hangs up--does the screen turn back on?