Vodacom - No power to you as a PAYING customer

I am so done with vodacom I keep on having problems with their debit orders. I dont understand it's like the over bill me each month and I dont understand because I have a top up contract. I phoned in to find out why there is an additional charge of R211 on my contract and at first I was calm I needed to understand this. After holding on the line for quite some time the agent eventually picks up again only to tell me that this was for airtime I purchased. I'm a bit confused here vodacom, when i purchase airtime without having any funds it automatically tells me there are insufficient funds to purchase the airtime. I'm so disgusted with their putrid service honestly! I don't even really want to use this number anymore because I always get **********. I have 2 contracts with you guys which initially was affordable but now it's like it's a ****, how do u put through airtime for a top up contract without funds!?? Does that even make sense???? 1st and last time I'll ever purchase contracts with you guys.