Over charged for cellphone contract

I have a contract with Vodacom which I pay R450 with insurance inclusive. Now the bill says I owe R786. When I call to find out why a sudden hike. They tell me about airtime advance which doesn't make sense because everytime I recharge they deduct the money I borrowed. I don't understand why they have to include it in my monthly subscription as I always pay back their money. So I have to pay the money I don't know about. Vodacom is a ****.