I bet it's a text scam - if so it's not Giffgaff taking it, if you accidentally signed up to one of those they can take it straight off your credit. You (or somebody else using your phone) probably clicked a bad link on Facebook or somewhere. They can read you number directly from their website and take your credit without any more imput on your part.
It should be illegal but at the moment it doesn't seem to be. There's some information here:
click here https://giffgaff.com/support/ask/incorrect_balance can take upto 24 hours
in the meantime:
delete unused apps and apps you dont really need as they all use them even when not open
disable mobile data when not using
ensure apps are set to only update via wifi also
how much is going missing at a time? £4.50?
if yes you have subscribed to a premium text messaging service, you need to send STOP to the 5 digit number that sent you the text message or follow info at the bottom of the message on how to cancel the subscription, it is not a gg charge
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