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If you need your PUK code, on an ATT Wireless phone (as opposed to a Go Phone or several other types, perhaps), go to
and then log in to your ATT account online, and get your PUK code there, then follow the instructions on this page, and enter the code carefully.
That's really it. If you find that you simply cannot obtain a PUK code (which goes with the SIM card on your phone, not the phone itself, and is not an 'unlock' for the phone, but just for the SIM card that is in the phone) you may have to obtain a new SIM card for the phone.
(If you have Google Contacts, of course, they are pretty much all stored online and you don't need the phone to transfer them to your new phone. But I presume you didn't use Google Contacts.)