LITERALLY ROBBING US OF MONEY WITHOUT EXPLANATION!

If I could give no star I would!!!! Seriously what terrible service. My mother has been with mtn for almost 5 years now, we decided last year to get a double phone contract and my sister was on her last year of a 2 year contract, so we have 3 phones we are paying, for some reason her phone price stays constant. Thank God. But EVERY SINGLE MONTH since we got our phones in July last year we have been charged a different amount than what it states CLEARLY in our contract. We have been to the shop 4 times already to sort this out. I have phoned, my mother has phoned. They keep saying they will contact us, and that they can see we are being billed inconsistently and said that would be put over into credit, yet a 11 months later we are no closer to sorting this issue out. My mother just left the mtn store an hour ago with same story, AGAIN. I have had it up to here now. My sister is due for an upgrade this week and is considering moving to a different provider due to this on going problem we cannot afford to NOT KNOW how much we need to budget for a month. Seriously inconsistent and inconvenient! If this issue is not rectified by tomorrow we are changing service Providers for my sister! There is no other explanation to this situation other than you are robbing us, clearly it is okay to ***** money that does not belong to you and cannot even give an explanation as to where it is going and for what. Honestly the worst service I've EVER recieved! I would appreciate this to be sorted out ASAP. Obviously the contract is not on my name, I share it with my mother. I expect a call regarding this situation TODAY! I wonder if I can go to the police station and report that you're constantly taking odd amounts of money from us without explanation, what then? Seriously makes me sick thag we have to keep running after you but you want our money and then take more than what is owed! Without an explanation. No No No