Vodacom data just 'disappears'

Vodacom data just seems to disappear into thin air. 75 MB data disappeared from my phone in a couple of hours yesterday despite the fact that I was on WiFi and not using the phone. This is not the first time, but the most drastic up to now. I do not use my phone for internet browsing, only for whatapp messages if not in WiFi range. When I phoned Vodacom I was told I was using the internet and used up all the data. I demanded to know what sites I was supposed to have visited. "It doesn't say, it just says internet," a Vodacom staffer told me. Then she told me maybe someone took my phone or the WIFI used my data!!!! The fact remains, I did not use my phone, my mobile data was off and the data 'disappeared'. I have heard hundreds of people complain about this and still Vodacom continues and is not in the least interested in even trying to assist customers. I am SO sorry I recently took out another contract with them. This is highly ********* and I really hope something can be done. Too many Vodacom customers are being bamboozled this way.