Refusing to give Paid Up Letter

My account was moved to legal without my knowledge. I paid that account immediately when I found out about it. However, this mishap has affected my credit record and I need a letter from MTN to state that my accounts are paid up, which they are refusing to give to me.

It is incredible that a company would not be willing to provide written evidence that my account is up to date. What a petty thing to hold against a customer. Unbelievable.