MTN wants to unduly benefit from a service they didn't provide and mess up my good credit record whilst at it

I had a 24 month cell phone contract with MTN. The end date for the contract was 30 Nov 2019. On the 8th of November 2019 I called MTN to inform them that I was not going to renew my contract with them after it ends in Nov, Palesa from MTN told me I need to call them back after the 24th so that the contract can be terminated. I called again on the 26th of Nov and spoke to Abrinah who even assisted me to change my SIM to a prepaid SIM. My sim card was then changed to a prepaid sim card since then and I have not been getting any data or minutes from MTN since. In June 2020 I found out from Vericred that I have been handed over for the very same account. I have been sending emails to their customer care and then keep on promising to come back to me and there’s no feedback to date. Now I am trying to buy a house and I am told that I am been handed over and there is no reply from MTN. Why do I owe MTN for a contract that has reached it's full term, SIM card changed by MTN to a prepaid SIM? Why are you messing my good credit record?