VODACOM USING MY AIRTIME FOR DATA CHARGES EVEN THOUGH I HAVE WIFI

Vodacom is taking airtime for data from my balance. I have WiFi so my data should not be deducted from my airtime. But I received an sms from Vodacom informing me that I am using out of bundle rates and funds went missing from my airtime. My WiFi is on and fully functional while this occurred. One can't even phone Vodacom and speak to an agent any longer because they have automated their options which are useless if they don't resolve an inquiry.