The Personal Unblock Kode (PUK) is the cure if you entered the SIM PIN incorrectly too many times (4). You have several (10?) attempts to enter the PUK correctly to unblock the SIM PIN. If you try too many times, the SIM is locked forever. Called PUKed. Some SIMs come with the PUK printed on the credit card sized carrier; other do not. If you don't have it, you'll have to call customer service to get it.