for a micro sim go to http://apps.giffgaff.com/microgaff/directrequest.php?r=chubuking
Contact an agent under My giffgaff. It probably has a bar on it as a precaution.
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I am sure I have read that replacement sims when the originals are lost have a bar on them that needs to be removed. Contact an agent and they will sort it as the sim will work in the o2 phone
it will still have a bar on it you will have to contact an agent to lift the bar