Mtn blacklisted me. WHY?

A client by mtn for years. And when you pay the account to close the black list you. I try to ask for help at mtn call senter and in store. No one can tell me how i can be blacklisted in june if i payed in october to settle them. All info i get is that i am by transunion up for R860. How is it possible. This do not make sense at all. I would like if they can sort it out