service cancelled in January

I cancelled my monthly billing to pay as you go early in January. I received an invoice from January, called 02, and was told to ignore it as it was incorrect and my account may well be in credit, following the cancellation of my monthly contract. To mention, my mobile has been switched off since the end of August, and has not been in use at all. I now receive a threatening letter about an outstanding amount on the account, which I find totally unacceptable. Please resolve this issue as a matter of urgency. This pertains to mobile number 07714 896372