A personal unblocking code (PUC), also known as a PIN unlock key (PUK), is used in 3GPP mobile phones to reset a personal identification number (PIN) that has been lost or forgotten.
Most mobile telephones offer the feature of PIN protection. After switching on the phone, if the PIN security function is active, the user is requested to enter a 4-8 digit PIN enabling the phone's non-emergency calling functions. If the wrong PIN is typed in more than three times, either the SIM card, the device, or both become locked. They can be returned to their original unlocked state by entering a PUC, provided by the service operator after verification. If the wrong PUC is entered ten times in a row, the device will become permanently blocked and unrecoverable, requiring a new SIM card. Cellular phone users are therefore advised by most providers to keep their PUC written down in a safe place separate from the device.
The PUC (PUK) is a SIM-specific code assigned and provided by the service provider.
Once you enter the PUK code, you must set a PIN. The PUK and PUK2 are used to unlock the PIN and PIN2 codes respectively if your SIM card is blocked.