Refusing to provide a Paid up letter

In 2016 I took a phone Contract with MTN and my account was handed over in 2018 to MTN attorneys which they demanded R6500 in oder to close the contract.

That didn't happen as they took my account back to the store without my knowledge as I was waiting for a paid up letter.

Fast forward 2021 I was contacted by Nue debt management about the same contract with a balance of R5450.I made arrangements to pay R500 with them via MTN stores but only to be told by MTN that my account has been closed 2019 May 11th and they even refunded me 1 month payment.

Now Nue debt claim that MTN should provide me with a paid up letter and MTN says Nue debt should provide me with a paid up letter.

This is affecting my credit life and I want to deal with this once and for all.