A PAC is a Porting Authorisation Code – it is used to transfer a phone number between SIM cards on different networks. The PAC is unique to your SIM card and you can request one from your network any time you plan on changing numbers.
Be aware that a PAC will expire after 30 days, so if you don’t change numbers within that time you’ll have to request a new one the next time you’re thinking about switching network.
Using a PAC is an easy way to transfer your number and the entire process is completely free providing you have fulfilled the contract with your old network.
The PAC code is only used to transfer numbers between different network operators – to transfer a number onto a new SIM from your existing network you’re instead going to need the SIM serial number for a SIM swap.