A PUK or 'Personal Unlocking Key' is a security feature on most mobile devices that protects your SIM card data. The PUK code is unique to your SIM card and is required when a SIM Card PIN code has been entered incorrectly multiple times.
How to get a PUK code
Register or log in to My EE.
Select Help & Support from the main menu options on your account homepage.
Select Get your PUK code.
Then select Get PUK1 code.
Once you have your PUK code, enter this into your phone to unblock your SIM.