If your phone isn’t working with a giffgaff SIM it may be locked to another network. It will usually say something like, “SIM card not valid”, “SIM lock”, or “Insert correct SIM card”.
Check if there’s a logo from another network printed on the phone
Some phones may display a message like, “SIM not accepted” when they’ve been barred as lost or stolen
Check out Unlockapedia for info on how to unlock your phone
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