Getting Locked SIM Message?

I went out and bought an iphone from Craigslist, and its running 3.1.3 and 5.12.01 . I have AT&T, and he said my sim card would work after connecting it to itunes. I did this and nothing works, when I put my SIM card into the phone and turn it on a message pops up that says "Locked SIM" and gives me the option to unlock or cancel. It gave me 3 tries, I used the first two guessing the defaults of 0-0-0-0 and 1-1-1-1 and now I don't know what to do. How can I get the code for my SIM? I called AT&T and they gave me the PUK code and that didnt work, even though I think that code is for once I used up my third try to unlock the SIM. Any advice on what I should do??
Thanks

This sounds like the SIM lock code.

Put the SIM back in the phone you had, it'll likely be locked. Go to an ATT store, they'll either unlock it or give you a new SIM for no charge.

Or, if you don't want to do that, to to ATT and tell them that you lost your phone. You have another phone you can use so you need a new SIM. They'll give you one for free.

The poster PM'd me about this issue. It seemed like the PUK worked in the end.
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