4G mobile data being used even when mobile data setting is off?
This has been happening throughout the world and is caused by a technical feature of 4G LTE. Most people don't even notice it because of the small amount of data that's used off their data plan. This is what happens: every time your phone receives or makes a voice call, it automatically switches from 4G to 3G mode due to the fact that LTE is a data-only technology. When the voice call is ended, the phone switches back to 4G mode. You may have mobile data disabled and restricted and yet this switching back and forth from 4G to 3G uses small amounts of data each time. You can prevent this from happening by setting network mode as WCDMA/GSM. So it seems that tech support guy knew exactly what he was talking about after all.