In order to be able to use your phone/tablet with giffgaff it will need to be fully
unlocked or an O2 supplied handset.
O2 configured phones will work on giffgaff without any problems.
This is because giffgaff is a Mobile Virtual Network Operator
(MVNO) which means that it uses someone elses network, (o2)
O2 and giffgaff are owned by the same company, Telefonica.
In some cases you may be able to get the original network supplier to unlock the phone for free,so try them first.!!
This is a guide to unlocking your mobile so you can use it on giffgaff.
If your phone does not appear on the list then the best option will be to have it unlocked at a local High Street store.