Do you know what you are talking about?

Received a very high bill which related to data usage. Unfortunately there is no way for me to figure out what this data was spent on. Find this very strange as I am almost always on wifi. All apps set to update only on wifi. Called the customer care call centre and was advised that even if I am on wifi they (MTN) will always first use my data limit and I should switch off my mobile data every time I am on wifi and switch it back on when I am not in a wifi area (say what???) Everyone I have spoken to says this is BS. Need an explanation please. Why is MTN "*****ing" my data? Hopefully this time someone will get back to me as promises to have someone contact me on prior occasions never materialized.