It was no problem for my Dad's replacement Lumia.
The screen kept going black, during calls (Abnormally, btw. Like you couldn't turn the screen on or hang up or anything. You just had to wait til the call was over and the screen would flash a couple of times, then it would come back on normally.) When received and trying to dial out. I tried a reset and that didn't do anything, so I called AT&T.
It was past the 14 day mark, and I told them the problem and they said they'd have one out to us the next business day.
It came the next day and they give you a return label and all to mail the defective one back. It was easy. And free.