Cell C tells me i owe them money after i settle the account

Cell C is the worse company I've ever dealt with, in fact they are *******. I had a contract with them, during covid had some financial issues and could not pay, so this year i called them to pay off the account, was told my account is handed over to shapiro shaik defries & associates, i called them and they gave me a settlement latter, i then paid the full amount on the settlement letter, I go back to cell c and ask for a paid up letter. Cell C tells me there is an outstanding balance on the account after i paid the settlement!!!