Unfairly Listed

On May 28 I received a message from Vodacom stating that they have handed me over to their legal team for an amount of close to R1100.00 which I supposedly owe to them. On May 29 I called them to make a query about that considering that I know I paid up the account in December and I was promised that they will get to me by June 1st which they failed to do. Today (July 23) I received another message from some legal team stating that I have been handed over to them and bearing in mind that my query has not been attended to. I feel that I am being unfairly listed and I need help in how I can go about clearing my name.