No paid up letter

I had a contract phone with MTN. I couldn't pay due to financial difficulties. Then MTN took me to MBD for them to collect the money i was owing them. First payment to MBD was made in September 2021 and the last payment in October 2021. I have been waiting for paid up letter up until today. There is another collection company that has started calling me from last month regarding the same issue. I have sent proof of payments and explained my situation to different consultants but i still receive calls regarding the same issue. May you please assist.