Cell C taking advantage of our ignorance

I've been purchasing the advertised R5 Watsup Data now R7.50 which promises to last for 30 days, but my normal data is still getting finished even if I do not use the phone for any internet activity. seems like Cell C is charging us just for the phone to be on and when I contact the Customer line no one is able to explain why and how this whole thing works. my Bill jumped from R63.00 to Over R1000 in less than 5 days and no one can explain how am I being charged