Clear my name

I paid up account more than 3 years ago, tried to apply for a contract yesterday and today they told me I still appear that im under collections, I called your call centre and was transferred to billing who said they cant do anything I must call your legal department, I was provided with two telephone numbers, which I both called and I was adviced that im by Nimble and that they don't have any MTN accounts under my ID number, I called your billing department again, who then advised they would send me to collections. Collections advised to call the same number, which refused because they advised to call them, I told them I cant be doing this back and forth, and the consultant hung up. I just want this collection off my name, I paid up long time ago, ive got a paid up letter as well.