Im being held to Ransom!!!!!

I cancelled my contract with Cell C in 2019 June and opened a new one. That line was supposed to be on prepaid and I continued with my new contract. Then I found out I have been blacklisted for none payment. Blake & Associates called me to collect on behalf of Cell C. I went to to store and paid the full the amount even though the contract was long cancelled and their agents agree this is anomaly. Its been two weeks now, both Cell C and Blake & Associates are failing to give me a paid up later... Can you just give me the letter so I can move on ....