Vodacom Sucks

Im very dissapointed in Vodacoms service. Im on contract and every month my account is debited, but i have now received two bills and one of them states that i owe Vodacom R877.00, why do i owe this when every month you company debits my account and when i run out of data i top up my own account. I have tried calling customer care and all i get is they under staffed and i wait on the line for hours as i need this query fixed asap. This is ridiculous.