Phone stolen after I sent it back to AT&T

Does anybody else have a story like this? I put my old phone into an envelope and mailed it back to AT&T. I was told I never received it, which I understand is not unusual after reading some of the comments here. But after investigation I'm told that they do indeed know who has the phone they know that it's active in my area. Inference being, I sold the phone to this person. Now there's no way to prove this except that AT&T knows what the phone number is, might even be a current AT&T customer using my stolen phone. Does anybody have any advice for me? Has anybody heard a similar story from AT&T?