So, I'm on my third iPhone due to the proximity sensor issue. The latest one also had the issue, it was getting really frustrating. I made a genius appt and went it. The genius checked out my phone and was able to replicate the problem. He then closed all the apps that were open and restarted the phone and tested again. After that he wasn't able to replicate the problem. He said that in some cases (not all, sometimes it is a hardware issue) if you have too many apps open, especially ones that are resource heavy apps open (like audio streaming apps and such, which I use a lot), this causes the proximity sensor issue. Its been a couple of days now and I try to keep not too many apps open and close most apps when making a call and haven't had the proximity sensor issue. Again this might not fix the issue on all phones, it might be a hardware issue on some, but it doesn't hurt to give this a try if you are having the issue.