Cell C IS MESSY

I called an Cell C customer care few months back to change my banking details for my two accounts. The agent captured one account, that led me my other account into arrears. I paid the amount that was due manually and they told me that my account is in order. Now recently I have been getting smses, calls and emails from Nudebt as Cell C has handed over my account, which apparently in arrears. I have been contacting Cell C to sort this mess out. They confirm that I have paid my accounts. Now I get threats from this debt collection company that I will blacklisted. I still struggle to get assistance. Cell C do not want to clear my name. Worst network when it comes to contracts. Lord Help.