Proximity sensor issue

If there was no physical cause for this problem like water damage or dropping it, ask Apple to send you an iPhone before you send in yours. They will put a hold on a credit card for the amount of the iPhone just to make sure you return the phone and it isn't broken to pieces.

They would have to replace the phone anyway.

But I doubt it would take a week. I you want to get our phone replaced without having a hold on your card, thy will probably overnight a prepaid box to you, overnight the problem phone to themselves, and overnight a new phone to you. 3-4 days max if you make sure to do your part in the morning/earliest time possible.

As for an easy fix... I highly doubt it.

 

No :-/

Apple likes to deal with customers needing support directly. That's how they keep their satisfaction rating so high. AT&T just ruins everything it touches so Apple wants as little interaction with them for customers as possible. I called AT&T tonight and the moron told me adding my iPad to my existing bill "can't be done". I did it two months ago and the next rep confirmed that there had been an iPad on my plan... But even she didn't know how to add it back... Lol. So much for their slogan "Rethink Possible".

You could try backing up restoring/DFU restoring the phone. That may work, but it sounds like a hardware problem to me. It's at least worth a shot and might save you a bunch of trouble.