MTN Top up billing - bad service

So disappointed in MTN once again - just wish they would listen and respond as they say.. I changed my account 2 months ago to a top up bill since MTN was charging me R 4000.00 per month. All went well this first month and all of the sudden my accounts changed with out my consent - I been told that my phone will have a R 550.00 amount and that i can top up the rest - now i get an outstanding bill of R 1000.00 AND STILL PHONE CHARGES... Even though i top up my phone any way - where is the other cost coming from ??? DON'T ANYONE OF YOU LISTEN ??? Please i need a superior or some-one to fix this as the consultants are to *********** to do this - My contract is finishing the 1ste December and i am so happy that i will be canceling my contact with MTN - DO NOT USE MTN they are the worse of them all.