Billed without my knowledge

my monthly instalment with cell c is R529, this month they go and decide to debit me R749. I took out the new contract in July or so where I was advised that I would get extra data for a 3/4 months. in January I called to find out about the data extender as I wanted to save the 2g that I still had but was advised I need to buy another 2g to keep the existing one. I figured in that case the data can just expire, which it did. so now when I call to enquire about the R749 being debited I am advised that it is because the data on my phone lasted past the month of January therefore I am being charged for what I used, and when I question why charge me when the data did expire and why was I not advised of this when I was taking out the contract? I requested to speak to the team leader where I was placed on hold for him/her but later transferred to rating the service. Well I got a call back from cell c but clearly there was a network issue and the team leader never attempted to call me back. this is my 4th contract with cell c and I must say I have had the most disgusting service since I signed up for this current contract, the S6 itself has stopped working.