Refusal to cancel unusable service

I tried to have an ADSL line installed at my premises in 2013. The lady on the phone asked if I would like a .... I honestly can't even remember what it was.... My current statement does not tell me either.... So I said yeah sure. So Telkom never came. My ADSL line was never installed. Yet every month I get an invoice for this service that I cannot use. And every month I phone from my cellphone (because I have no landline....) to cancel this service and every month they say they will cancel it for me and credit my account since they are giving me a service that was supposed to come with an ADSL line. I never received the ADSL line. How can you justify billing a person for an addon that he cannot use.... So Today I received a letter of demand for the R2352.25. Yet last month after hours of getting the runaround I finally had a person tell me that they have cancelled the account and credited the amount in full. So why is Telkom still harassing me for a service that I could never have used in the first place.... I'm not going to pay this as the service was supposed to come with the ADSL line. Since I never received an ADSL line from them they need to reverse these charges...