Telkom customer service has ignored my requests for a year

I had a Telkom account - for uncapped ADSL. At a point i became unsatisfied with Talkom's service and attempted to close the account. It was then brought to my attention many months later that the account was actually never closed and that I had to pay an outstanding balance. So I paid the outstanding balance - I think it was about R700. I don't have proof of this anymore. It happened over a year ago. My problem now is that i am being harassed by a credit collector daily saying that I owe Telkom money. I do not have any recollection of making an agreement with Telkom that they could hand over debt. Frankly, the way I am being treated by the credit collector is disgusting and I am holding Telkom accountable for that treatment. I do not want to receive these harassing calls any more, and furthermore I do not want to pay anything extra to a credit collector who I have no agreement with. Now I've been trying to sort this out with Telkom for a year. We have a Twitter correspondence going back to 14 September, but every time they promise to get back to me nothing happens. This has gone on way too long.