Incorrect Invoices monthly

every time i receive an invoice from Vodacom its in correct, I need clarity as this issue has been dealt with before & I always get replies that all is been correct going forward, but come the next invoice, its not, the invoice is always more that what we agreed on. another thing is the contract agree entered into, I've never even used any of the data package provided to me from inception to current date, due to the fact that I've misplaced the sim-card & always when i call their customer care line, no one can assist with getting new sim-card, but they keep on debiting my account every months without assisting me. very disappointed by their service, it might be little money to them, but to be its a lot...