MTN ruined my ITC score

I took a contract with MTN in 2019, once my contract ended in 2021 I called them to inform them that I do not wish to continue with the contract but want to be put on prepaid. The consultant agreed and the call ended. Now I want to buy a house I can’t because MTN has tarnished my credit record saying that I owe them payments of 6 months. How is that possible ? When I check my e-mail I see they have been sending me statements of R2 every month, I don’t know what they are charging me for because there’s nothing that I get from them. MTN please clear my record, my life cannot stop because of your incompetence.