Paid up letter

I have been handed over to dent collectors last year and settled the account last year August 2023 but only to find out that I have listed on credit beareau as a default account whereby I did settle the accouny, MTN advised me that I still owe them and I payed them what was outstanding and still to date have not received my paid up letter from MTN , they advise me that I will get it after 31days ? Iam really not happy with MTN legal department they just don't care they never even looked up my query