Account not closed

On the 3rd of november 2021 i called these debt collectors and requested a settlement letter for my OLD MUTUAL account which i received. I settled the account within 3 days. To my surprise, a few days later i received an email about my account still being in arrears. I requested them to send me a paid up letter when i sent them the proof of payment but till today i have not received it. Today i get an sms that the same old mutual account i had paid with them is now sold to another collection company that i also have to pay. I need help