Loading an additional airtime balance

I got a contract on an iPhone 5s in mid December and as per my conversation with the sales lady, I would be debited for Dec and Jan in January 2015 understandably. And R350 every month going forward, now I owe R849, R500 airtime that I did not know about or ask for. This is deceiving customers I don't have R500 laying around and the call centre is not even helpful telling me that I used the airtime. I thought I was using my 100 minutes and didn't keep tabs because I was unaware of the additional airtime added to my contract. This is unacceptable and needs to be addressed