Been overcharged for a service I never requested

I have a 100 topup contract with MTN. I upgraded my previous contract in May 2015 and in November 2015 my plan was changed to an open line without my permission. I queried this in Dec, Jan and Feb and at first noone could explain what the problem was and why I was overcharged. Finally in February someone explained properly and said they will investigate and credit me back what was overcharged. In March I was overcharged again! When I called yesterday to find out why they said there is no credit due to me. Nevermind the fact that I never requested my plan to be changed to begin with. 5 Months of such inconvenience with no resolve. Noone gets back to you regarding a query, you have to call them all the time or your problem will never be resolved. I am very unhappy about this!