I have the SAME EXACT thing going on with my phone. You're right. It's not a sensor. It's after you set phone down for a while when screen goes dark. Then when someone calls, screen stays dark. There's nothing to swipe to answer it. U have to wait till they hang up. Then you can go back in and redial their number. If you are already messing with your phone before the screen goes dark, there's no problem answering it. It's only after there hasn't been any use for a few minutes when it goes dark that it stays dark on an incoming call no matter what buttons u push or even the on/ off button. It's very frustrating when you can't answer the phone. They need to fix this problem with the Androids.

My screen does the same thing. It stays black when someone tries to call me. I can't answer it. I tried swiping screen and pushing button on the side. I am only call the person back after they are done ringing. Then I can go back into my phone and try to redial them. I have Android phone. Why does this happen? Frustrating that I can't answer an incoming call.