Do you have WiFi calling enabled? Voice uses about 1mb a min, video/facetime about 7mbs a min..
I’m not sure, but my wife may have wifi calling enabled.
Generally it would work out fine as our DSL lines are only throttled by the instantaneous data rate limit.
It would save on the (low) 3rd party cell phone account monthly data limit, so I may have had her turn it on, if available.