Why does data roaming work on IPhone 6 even when it is turned off

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We have an area where we can't get data from our provider without roaming. If you turn off data roaming and just have on voice only roaming then you still get data. If you turn off roaming altogether then you don't get data. That would mean with data roaming off we are still roaming. How do you turn off data roaming without turning off roaming altogether because if you turn off voice only roaming you can't get calls either. Iphone 6