It's linked to your SIM card. Contact your network provider.
PUK code kicks in when the PIN code has been entered incorrectly 3 times. Insert the PUK wrong incorrectly 10 times and the SIM card is permanently disabled.
You two should be ashamed. Instantly jumping to Holier-than-thou conclusions = fail.
I came to this forum with the exact same problem, because I put the sim in an iphone 4, tested out facetime (which works flawlessly btw), tested out text and voice (which both fail), and then fooled around in the settings.
I saw the sim pin menu and, not knowing what it would do, swiped sim pin to on, when I did this it said, "Enter pin". Well, not knowing what it would be for an ipad, I just clicked 'O.K.'. The message didn't go away... so I clicked ok again, and accidentally double-tapped, for a total of Three tries. Now I'm sim locked in both the phone and my ipad. At&t isn't open right now and I'd really like to have the thing back working tonight before I have to leave in the morning.
Thanks Cap, yeah I found out last night after I acccidentally locked it that the default code is normally 1111. They shouldn't make it to where hitting the O.K. button with a blank number counts as a try.