I have been in the cellular industry for over 10 years with Verizon, AT&T, T-Mobile and Sprint. I will...
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"A PUK (PIN Unlock Key) code is required to unlock SIM cards that have become locked following three successive incorrect PIN entries. If your SIM card becomes locked, you will need to get a PUK code from AT&T to unlock your SIM card.
You can obtain your phone's PUK code from AT&T in one of these ways:
Log in to your myAT&T online account
Use our Technical Support Chat
Contact Customer Care at 800.331.0500
Find the PUK Code for your phone online
Go to the My Phone/Device page in myAT&T.
From the My Phone/Device screen, select the Unblock SIM Card link under Manage my phone/device.
The Unblock SIM screen will display your PUK code and will give you instructions for unblocking your SIM Card.
Once you receive your PUK code, you will need to enter it into your phone to set a new PIN code.
If you enter the wrong PUK code 10 times in a row, your SIM card will be invalidated, and will display an error message: PUK ..."