5 gig data used in a week / impossible

Again 5 gig in a week .. This isn't right note called for ... My dad is a pensioner , uses cpu maybe once a week<br> For email .. Mom and him have wifi on phones yet they hardly use it ... It comes to my attention that doesn't matter what company at the moment - it's as if those with pay as you go are being stollen from ... Folks don't download or use apps and still data is gone - and never does anyone come back with info that can justify the data usage .. Just same old , same old - check setting and so forth pass the buck ...