Personal unblocking key

A personal unblocking key (PUK), sometimes called personal unblocking code (PUC), is used in 3GPP mobile phones to reset a personal identification number (PIN) that has been lost or forgotten.
Most mobile phones offer the feature of PIN protection. After switching on the phone, if the PIN security function is active, the user is required to enter a 4-8 digit PIN to enable the phone's non-emergency calling functions. If the wrong PIN is entered 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 PUK provided by the mobile service provider, after verification. If the wrong PUK is entered eight times in a row, the device will become permanently blocked and unrecoverable, requiring a new SIM card. Mobile phone users are therefore advised by most providers to keep their PUK written down in a safe place separate from the device.
After the PUK code is entered, the PIN must be reset. The PUK are used to unlock the PIN codes if the SIM card is blocked.