Vodacom wrongfully billing me

I was behind with my account so decided to make arrangements to settle the account. So the arrangement was made last year December. The agreement was I will pay for 6 months, did that all good and well. Came end of my arrangement which was this May. Then all of a sudden I had other amount outstanding which they can't explain to me. Decide to just pay the amount. Now again there is an outstanding amount that they cannot explain to me I am tried of vodacom reaping me off. When you call the call center you are taken of one department to the next till, today no can tell me how I accumulated that amount.