Not upholding contract and bad customer service

I've had a 24 month contract with Vodacom. Randomly in April, they changed my contract, randomly addding more data and charging me more without my consent. Upon querying this, I wans very politely told that that's the way it is and to suck it up and pay. Not keeping in mind that I have a legal contract. Then since August is my last installment on my contract, my installment was all of a sudden higher again. I queries once again. It was logged and the query was closed without anyone getting back to me as to why that is. I had to phone again only to be told that they coudn't answer me. It was the back office was the answer but yet we have to deal with customer service that cannot provide service? Or advise why back office did what they did. There is no reason given as to why my account is more or why my contract was changed without my consent. Normally I have only had good dealings with Vodacom but lately that has changed and I will most definitely be changeing my service provider!