I'm having the same issue and have gotten tired of hanging up on people. As an experiment I called voice mail several times while placing my hand on different parts of the screen. At the beginning of the calls, placing my hand most anywhere on the top half of the screen correctly turns it off. About a minute into each call my screen would wake up regardless of where my hand is and then the screen randomly turns on and off. Sooner or later my cheek must be hitting the lit screen in the right spot, and I've hung up on someone. Again. I downloaded an app called androsensor from the play store and it seems to confirm the problem. The proximity sensor only consistently works properly when my hand is near the very upper left part of the screen , and even there, it eventually fails. I'm going to try a factory reset, and if that doesn't fix the problem, I'll have to do a warranty swap. I've now hung up on my boss, and my SO several times and I'm not sure they are buying my "defective phone excuse" any more.
I should also note that removing the bumper and cleaning the screen failed to fix the problem.