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It would be beneficial to take a look in your myvodafone app or myvodafone online version as this should give you an idea what the credit has been used for @Demihuggett1991
There are two Vodafone Payg options to which you'll be one one of them.
The Big Value Bundle option which takes Credit from your account every 30 days to pay for your next set of allowances.
And then there is the Payg1 which has a daily cost as explained in Vodafone-pay-as-you-go-1.
Other usage that can take your Credit are MMS Messages i.e picture messages and if an unwanted subscription has happened - see further information in Premium-rate-SMS-FAQ and Vodafone have a Premium Rate SMS Bar through your myvodafone or a person uses up all their allowances and start to pay the basic Payg charges.
Live Chat or Customer Services on 191 can access your account to help check what's used your Credit.
If needed the Vodafone Social Media Teams via Contact-us-for-account-specific-queries can too. Link back to your thread here including your forum username so your not having to repeat yourself again. I'd suggest Twitter.
I wish you all the best with this situation.
🌈Stay Safe & Stay Alert. 🌈
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