Incorrectly billed

I have been over billed for the past 4 months, each time i call to rectify this, all i'm told is that a credit will be created on your account, and that i will then be billed the correct amount of R588. R588 is how much i am supposed to be billed, so my question is, where is the difference that i have already paid, as this is money that i should be refunded with. I have yet received a call and have yet received the money owed to me. The first person i called told me that the credit would take my next payment to R588 less what i was billed, this has not happened, instead i have still being billed incorrectely.<br> <br> I called in to cancel my contract as i am clearly being **********, however i have not received any call backs. Why should i pay a cancellation fee when money is being deducted from my account *******ly.