1) Find and buy a new deal from O2
Your next task is to choose and sign up for a new deal contract from O2. Once you’ve passed O2's credit checks, you’ll receive your phone or SIM card in the post.
2) Contact O2 to give them your PAC code
Now you’ll just to ring O2 to give them your PAC. You can get in touch with O2's customer support team on 034 4809 0202 from a landline (standard network rates apply).
3) Insert your new SIM card from O2
If you got a new phone, use your pin-key to unlock the handset's SIM tray. Then insert your O2 SIM.
If you went for a SIM-only deal, you can just put the O2 SIM in your existing phone.
While your number is being transferred, you'll be supplied with a temporary number to use. So you can still make and take calls and do whatever you'd normally do on your phone.
4) Wait for your number to be transferred from O2
The transfer should be complete the next working day. But this varies depending on the network you're joining.
The transfer could also take a little bit longer if a weekend or Bank Holiday gets in the way.