A PIN Unlock Code (PUC), also known as a PIN Unlock Key (PUK), is used in GSM mobile phones and some smartcards to unblock a blocked card.
Most mobile telephones offer the feature of personal identification number (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 reverted to their original unlocked state by entering a PUC, provided by the service operator through 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 by the service provider. It is near-impossible to guess and enter it; thus, if a user does not know his or her PUC and is prompted for it, it should be obtained from the service provider.
Once you enter the PUK code, you must set a pin. The PUK (Personal Unblocking Key) and PUK2 are used to unlock the PIN and PIN 2 codes respectively if your SIM card is blocked.[1]