MTN is ruining my credit record

I took out a contract with MTN in 2019 August for the duration of 24 months. The contract ended in 2021 and i went and terminated and converted my contract number to be a prepaid number. MTN continued debiting my account for a lesser amount and it took me some time before noticing. i then started reversing my debit orders with the understanding that i no longer have a contract with them. Recently i checked my credit score only to discover that i owe MTN a lot of money. i tried engaging with their legal department and gave them the reference number as proof for my cancellation but all they say is that i owe them without them issuing a statement or a breakdown of what i owe. MTN is ruining my credit record. MTN is a ****, they can't expect me to pay for a service i am no longer using/ receiving. i have never missed a payment and i have proof.