The problem occurred because customers who bought a Giffgaff "goodybag" bundle - comprising voice and text minutes as well as data allowances - using their prepaid credit, were overcharged.

Giffgaff works in a different way from most networks. It doesn't offer traditional phone contracts – instead, it sells monthly pay-as-you-go (PAYG) Sim cards that you top up with credit or bundles of ...