iPhone 4s ATT not activating, even with ATT sim

Hi, My brother bought 2 iphone 4s 32gb's off of someone (craigslist). Both with box and all accessories and it seemed legit. The one 4s works great. The other one is stuck at the activation screen. I have plenty of non-active ATT sims that have always worked for activating iphones...but it will not work in this phone. It says SIM NOT COMPATIBLE. I also tried my active straight talk sim and it says the same thing, and I've activated 4s's before with that sim as well.

So, I called ATT. They checked the IMEI blacklist database and said the phone was not blacklisted. But then they said the IMEI doesn't look like its in there database at all. They refused to acknowledge that it was an ATT iphone, and even said it must be a different carrier. I called Apple and they said that it was indeed an ATT iphone, but I tried to explain that to ATT and they didn't seem to get it.

Finally, I tried unlocking the IMEI via unlockfusion (ebay), and they say the phone is not an ATT phone (but it IS an ATT phone, apple themself acknowledges this, but for some reason the IMEI is not in there database).

I won't have my car back for a few days, but I do plan on taking this into an ATT store, then into the apple store. I just wanted to ask if anybody had any advice. I do believe that this phone has been used on ATT before, but can't confirm that, but it does have some wear.

Thanks a lot for any help!