A PIN Unlock Code (PUC) or 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 3 times, either the SIM card, the device or both become locked. They can be reverted to their original unlocked state by entering a PUC (PUK), provided by the service operator through verification.
If the wrong PUC (PUK) is entered 10 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 (PUK) 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 you do not know your PUC (PUK) and are prompted for it, you should get it from your service provider.
Please note (for Indian customers): You may directly contact the service provider and get the PUK number. For this you just need to have the mobile number.
Also read post on mobile phone locked/ forgot phone unlocking password