Transfer my number to giffgaff
It's a piece of cake to bring your old number to giffgaff. And you can transfer any mobile number from any UK mobile provider (though they may charge you if you have a contract and leave before it ends).
What you need to do:
Request your Porting Authorisation Code (PAC) from your previous mobile provider.
Once you have your PAC and an active giffgaff SIM, come back here and complete the number transfer form (which will appear by magic once your giffgaff SIM is active).
Log in now to bring your old number to giffgaff.
Important note: On the day of transfer you'll experience a loss of service during the transfer process. You may not be able to use either your old number or your giffgaff number for serveral hours up to 9pm on the day the number is transferring. Turn your phone on and off throughout the day.