MTN billing issues overcharging

I have an account query, I have been over charged on a contract that I took out 6 months ago. I called the call center to sort out the incorrect billing, they told me I need to speak to someone in a store as they sold me the contract... I argued that my issue is accounts related but still I was told to go into a store. Eventually the call was ended by the MTN agent. I got no ref number. I then decided to go in store only to be told by the sales agent that I need to call the call center with them... my issue is still not sorted out while I am ****** every month. It's not alot of money but it's the principal. How many other customers do they do the same thing... absolutely poor customer service. They think they are too big to fail.