Unable to fully switch off data roaming on iPhone 7
On a recent trip I incurred roaming charges despite having all data roaming switched off. My network has advised that "regarding the data while roaming charges, please note that it is mainly caused by some LTE handsets designed to send and receive minor data sessions even though mobile data is turned off."
Is this correct and how can I avoid future charges when travelling?
iPhone 7, iOS 11.2.5