Are you using the SIM in the UK? There is a lot of confusion with the new EU roaming directive especially with the ‘tag line’ ‘roam like at home’. You cannot roam like at home as use in your ‘home’ country is not roaming.
The EU directive ONLY applies to use of your SIM outside your home country it does not legislate for charges while using a UK SIM while in the UK
If you are calling from the UK you are not roaming and normal international call charges apply.
To call internationally from the UK you still, as you always did, require airtime credit from www.giffgaff.com/buy/topup