A PAC code, which stands for "Porting Authorisation Code," is a unique code issued by your mobile network provider that allows you to transfer your phone number to a different service provider. It's used in the United Kingdom and enables customers to switch mobile phone operators while keeping their existing phone number. When you request a PAC code from your current mobile operator, they'll text you the code along with other important information such as your remaining contract duration, charges for leaving early, and the date the transfer will take place. You then provide this code to your new mobile operator to start the process of transferring your number to their network.