When on a call and you move phone away from ear, the screen should come back on. Note 3 is having issues with proximity sensor, and you have to press power button to get screen back on. I have read about it on other forums as well. People talking about blowing air on the speaker port, but the only thing I have found to work is press hard on the ear speaker area. It will temporarily fix the issue, but it comes back. Definitely a hardware issue, but hoping they can fix it with software update.
Let us know if any other "fixes" available.