OVERCHARGING

Data charges runnning over R800 for the last few months....when constantly connected on wifi, at work and at home. How is this possible....call centre people giving an answer saying that mobile data should be turned off when connected to wifi otherwise it will still take phones data.....how come this doesnt work the same on other networks???? And why is the data charges so high only the last few months and not straight from the start....please do explain! This is unacceptable. We have been requesting itemized billing for 2 months now, and still nothing!!!