It sounds like you have a monthly payment plan and billed quarterly, it's an old way of paying and causes confusion.
When your last bill was generated, your payments after will show as a credit due to no balance to pay, when your billing cycle runs again it will take the credit towards the bill charge.
£351 is a large bill, I'd honestly recomend speaking with the loyalty team, ask for an account review to check you have the right calling plan for your needs, they can also change you to a whole bill direct debit and change to monthly payments, it takes all the guess work away, the amount of your bill is how much is taken by direct debit, so you'll never have debt or be in credit again.
The number is 0800 800 150 - when the IVR asks why you're calling say 'loyalty team'
