HTC One blank screen problem

[jackpete]

When I am talking on the phone and the screen goes to sleep, it will not come back on when I move the phone. If I hit the on/off button, it hangs up. If I need to press a number and the screen is blank, I have a problem. I just had a supervisor join a recent call and when she joined the call, my screen went on and I was able to hit answer, but otherwise I seem to be stuck. Is this a proximity sensor issue or a simple setting? I can't find anything in the settings. I tried setting my sleep to 10 minutes, but it made no difference. Thanks for any help (HTC One, Credo (on the Sprint network))