You’ll need to call your current network operator to request a PAC code for your SIM card. Some networks, such as giffgaff, have an online service where you can request a PAC but calling your network is often the easier option.
The process of requesting a PAC is usually automated, so you’ll have to navigate to the relevant section of your network’s automated answering service. If that option isn’t available you can always talk to a human operator and request a PAC from there.
It may take up to two working days to receive your PAC and it will expire 30 days after you requested it.
Your PAC is normally sent to you via text message but it’s a good idea to make a note of it separately as you will need to have it to hand when you’re sorting out the transfer with your new network.