Mobile data turned off but having usage charged

Thanks Brett. It's helpful. I don't have Exchange ActiveSync account. I always set fetch data for apps manually. I always turn off cellular data unless I need to use it because most of the time there is WiFi connection in my environment.

But I still have a question. On the article "Learn about cellular data settings and usage on your iPhone and iPad (Cellular Model) - Apple Support", it says:

"When your device is connected to Wi-Fi, any FaceTime Video or Audio calls you make will use the Wi-Fi network. If your router loses its Internet connection, or if the Wi-Fi connection degrades, you'll stay connected to the Wi-Fi network, but your device will route the data over cellular data."

Does this mean even if the cellular data is turned off, this will still be the case?

To check where could be the cause of data usage, I turned off every thing: cellular data, FaceTime, iMessge, all background app refresh, all applications from using cellular data. But the data network was still charged. When I switched back to iPhone 4S with same setup, it was fine, no data usage occurred. So it must be the problem with iPhone 6 plus. I don't know what else I can do. It's eating my monthly mobile coverage.

Thanks Brett. I've already contacted them. Hopefully the problem can be solved.