Hello there! So I have had a similar experience with the Note 2. It is not hardware related as far as I can determine because I swapped out the prox sensor on mine and it did not help. I feel that it is kernel related. On my previous Note 2, I flashed some kernel, prox sensor stopped working. I flashed back to stock AT&T and it worked for one call before quitting again. That's when I tried a new sensor and camera unit. The phone was under warranty so I triangled away and traded it. With the current Note 2, I flashed Sky note 9.3 and then after a while the prox sensor stopped working, I just thought okay with whatever maybe it's the kernel I'm using. I wanted to upgrade to Sky note Air, so I did. Got all settled in and decided to see if their provided kernel fixed my call sensor, no cigar. So I went into *#0*# and realized that none of my sensors were working. Tried using Devil Kernel, stock, stock ROM, all sorts of configurations and I can't revive any of them. Feeling very defeated.