I have requested info from the company (iburst)
Had a similar experience with them as well. They threatened me with black listing and we had a serious difference of opinion regards the bill as i had cancelled the service and they continued to bill. Wanted to sell me a new modem when i changed to Vista as the original didn't have a patch, i refused to do so.

Also Telkom have tried ... they sold there debtors on and this causes a whole world of trouble. I paid the outstanding bill with the first collector. About a year later ANOTHER collector tried to collect same account with higher interest. About 6 months ago it started again with yet a different collector.

The way i resolved them both is EVERYTHING in writing, email, fax etc. No phone conversations ASIDE from the initial call. Requested statements, payments that were made etc also advised them Black listing without these and we would have them in court etc. Once you listed then to late i guess but try to pick it up at the start.