Billed on a contract I cancelled years ago

I previously took out a 24 month contract for my internet ****le which was debited from my account. Having not realised my contract had reached its end I continued to pay on debit order for a long period after my contract expired. By the time I realised this I had accumulated a considerable credit on my account. I called to cancel my contract and was told that as they couldn't refund me my credit they would cancel the contract but keep my telephone number active until such time as my account reaches zero, at which stage my telephone number would become inactive. I requested this is writing but was told it was not necessary. As my debit orders then stopped I assumed all was in order.<br> <br> For the last few months I never received an invoice showing my credit, despite numerous requests.<br> <br> You can imagine my surprise and horror when I EVENTUALLY received an invoice stating that I owe R5K. I have tried calling the accounts dept and legal dept on numerous occasions but am getting nowhere. <br> <br> I am beyond dumbstruck as to how I could be billed on a contract that I cancelled years ago!!!