Airtime

Could someone please explain to me what would be using up my airtime when my data is switched off? I use MiFi or WiFi for Internet. I thought I was imagining this until this month. I made a call for the first this month this afternoon that lasted 22 seconds because I ran out of airtime. So please break it down for me Vodacom as to how does one use up their airtime with their data switched off and yet to make a call? We pay you more than enough for our data and airtime for you to still have the audacity to ***** it back. Can I have my airtime back please.