TOP UP SUDDENLY OPEN LINE

It is not the 1st time happening. I was not going to renew my contract because of the problem the last time and yet I did after so many promises. I made it clear to the person who handeld my contract i want a top up line because i cant afford suprises. The contract has been fine for a few months bug suddenly this month my data was open and i had to pay double then usually. I phone all i get is always the credit ia being corrected and the money is goin to be in your account. The lady at Montana branch is the only one giving feedback and helping me. It is really pethetic how a airtime line suddenly becomes an open line after i made it clear i cant afford an open line.<br> please head office do something because i really dont unserstand how its fair! I have a vodacom line and when airtime is finished its finished i cant do nothing till I buy airtime!