i did have a recent bad experience with unlocking phones i would like to bring up. i purchased an amazon fire phone from eBay. i was able to get my money back from the seller. but the phone came back bad. AT&T told me that the phone was "not intended to be used on our network" they wouldn't tell me why. they told me the phone was a test model. or maybe it was a store demo. or it was some kind of prototype. i asked them if they wanted their phone back, they refused.
i asked them for an unlock. they would not unlock the handset. they would not sell me a line of service. i called amazon for help. they were unable to unlock the phone. they said all they could do is talk to AT&T. the phone was sold to AT&T and they were in charge of unlocking it.
what i learned from this experience is do not trust imei checkers 100 %. if you want to check an imei, then go to at&t's bring your own device page. if the imei # is bad, they won't let you add a sim card to your cart.
amazon did not stand behind their product. they did not offer to swap me