Incorrect Billing

I took out a contract with MTN for data after my old contract had come to an end. i was informed that my old contract had been terminated and that the new contract would be on a month to month basis. this has turned out to NOT be the case as i am currently being billed a "promo service fee", this is a fee that is billed to customers who have received a device upon taking out a new contract. My new contract did not include a device but i am being billed for one! I have called the MTN customer care center and logged several requests to have this fee removed from my contract but i have had little to no assistance. I have resorted to witholding payment until this very simple and uncomplicated issue has been resolved.