Personal unblock keys are codes for resetting the personal identification number of a cellphone's subscriber identity module card. Cellphones with the PIN protection feature require users to enter a PIN to unlock the SIM card before they can access the cellular service. This PIN is changeable by users.
If a customer enters an incorrect PIN three times in a row, the SIM card becomes locked. To unlock the SIM card, he needs to enter a SIM-specific PUK assigned by his cellphone service provider. This code is unchangeable, and a SIM card becomes permanently disabled if a user enters an incorrect PUK ten times in a row.