Horrified of the Service

I had 2 contracts with MTN, one for Data and one for a device, the one for a device was coming to an end and I decided to not do an upgrade rather cancel, I then initiated a cancellation of the 3rd of April which was completed and was advised to pay a final amount by the 3rd of May.. all well and good payment done. the one contract that i now have remaining for Data MTN did not debit my account as normal, I then decided to call the contact center to check why I was not debited and how much I needed to pay if I were to make a Cash payment, the person that I spoke to said to me that I owed R11000+ for early cancellation of my device contract, my question to him was How come because my contract had already been paid up and cancelled, the guy could not give me an answer and hung up the call, called a different person, the lady logged a query for me last week Thursday and then advised I could do a follow up on Saturday, when I followed up, No joy... No one can tell me what is going on... Is this really how a customer should be treated? I do not owe them this amount and this makes absolutely no sense to me that no one at MTN can help me!