The PIN unblocking key (PUK; sometimes called personal unlocking key) is a master security code for unblocking and resetting the personal identification number (PIN) for your SIM card. For added security, the PUK is unique to your SIM card.
If your PIN code (default is 1111) is active and you enter it incorrectly three times, your SIM card will be blocked. You won't be able to use your phone, except for emergency calls. You must enter your PUK into your phone to unblock the SIM card.
You may receive this error message on your phone "SIM card locked"