CELL C ACCOUNTS DONT KNOW WHAT A RECONSILING ITEM

Cell C invoiced me two years ago for R6222.84 for data used (in one week) After several phone calls, I pay tem the money. Another phone call from them where they claim that I Still own them R896 I asked the lady there if this was the final amount to pay to get them off my back and she confirm the amount. I paid it. They handed my account over to a debt company Nudebt and claim another amount of R249.17 Which I paid.... What can I do to get them off my back for ever. I keep on sending proof of payments to both companies Cell C and Nudebt but they keep on sending this sms's threatening me with black listing. I also asked them to listen to our telefonical conversations. Please, anyone with advice will be welcome What do I do should they black list me.