Being charged for business lines I cancelled

I am not very happy with the service I have received from Telkom Liberty Midlands Mall. I owned a company and had an ISDN line ***) and a ASDL line ***). When my business closed at the end of August 2014, I phoned to cancel my business lines and was told that I need to submit a 30 day notice written request on a company letterhead. I did this at the beginning of September at the Liberty Midlands Mall branch. I received an invoice in October which I paid but then kept receiving invoices for a service I was no longer using. I again went into the branch only to find out that the cancellation letter was still sitting on someones desk and hadn't been faxed to the relevant department. A lady by the name of Zama assisted me and assured me it would be done. I am still receiving monthly invoices and now Telkom wants to take legal action as they haven't been paid. I again went into the branch as you can't phone them - they don't have a landline number. No one was able to assist me. They couldn't locate my cancellation letter and the system tells them that only one line was cancelled. I refuse to pay for Telkoms error and for a service that is not being used.