When the personal unlock key, or PUK, number is entered incorrectly 10 times on an AT&T phone, the SIM card will be locked. When the SIM is locked, the "PUK blocked call operator" message is displayed.
The PUK is part of a security feature designed to protect the customer's phone. The SIM card lock is permanent and cannot be removed. A new SIM card must be purchased.
To prevent locking the SIM card, a customer can contact AT&T's customer service and request the PIN code for the SIM card. Once the code is entered, the phone is accessible again and ready for use.