Charged for airtime advance that I don't use

It's the second time I complain about being charged for airtime advance while I didn't use it. For the past 2 months vodacom has been charging airtime advance on my contract account while I didn't use it. I spoke to one of the consultants who confirmed that the airtime was paid up and I didn't owe vodacom anything and she didn't understand why I was being charged. She promised that the account will be credited with the same amount but nothing has been done. My normal account shout be R677 but vodacom charges me R738. Really??? And this is the second time