Handed Over to Debt Collectors after my Account was sorted out ( took 9 months)

After 9 Months Telkom cancelled and rectified my account with them and even send me the invoices to show that my account was cleared and I do not owe them anything. Now I received a sms from Debt collecters (Nudebt Trust) that I must pay the amount of R2785.57 sent in there account in the next 48 hours or they are going to blacklist me. The account was settled or cleared on my side but was then handed over to Debt Collectors. Can somebody please tell me why?