This should explain it for you :
A PUK code (Pin Unlock Key) is required when you enter an incorrect PIN
3 times. After entering the wrong PIN 3 times, the card will be locked
and you will need to get a PUK code from AT&T to unlock it.
You can receive your phone's PUK code from AT&T, either by logging into your account at
myWireless Account, visiting our
Technical Support Chat, or by calling Customer Care at 1-800-331-0500.
Once your PUK has been received, it will need to be entered into your phone to set a new PIN code.
If the wrong PUK code is entered ten times in a row, the SIM card becomes invalid and you will need to
purchase a new SIM Card. The phone will display "PUK blocked call operator".
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