I have an iPhone 6 running ios 9.1 with Verizon. Recently I turned on the option for Voice and Data over LTE (the default is data only over LTE). Since turning on Voice and Data over LTE, during many calls the entire screen turns black (like the proximity sensor), but when this happens there is no way to recover (or do anything on phone) until the other party hangs up. Home button and any other buttons, cease working. In order to get the phone functional again, I have to do a soft reset. (Press and hold the Home button (big circle below the screen) and the Sleep/Wake button (on top of the iPhone) simultaneously for 10-20 seconds. Release the home button until it resets (you’ll see the Apple logo), then release the power button.)
Again, this only happens when Voice and Data over LTE are enabled.