billing

I have cancelled my contract in September and now when I want to apply for a new contract I'm told there is an outstanding amount due to them while I called a couple of times to get contract cancelled. My worry is if I didn't consider applying for a contract I was going to discover later that mtn has blacklisted me for something I don't know. I'm really shocked by the service they have been providing me as a client who has forever been loyal to them.