R 270 suddenly gone because "mobile data"

<p>Yesterday I used my fnb online app to buy R270 worth of airtime. When I am at work I am automatically logged onto the work wifi and I generally am too busy to do anything besides checking a few notifications and the time. When I leave work I try to make a call and "nothing". When I get to my destination I call vodacom and I am told that the airtime was depleted because my mobile data was on. This is very confusing because its not as if I was updating apps or watching videos. How does R270 disappear because of mobile data being on when it has never done this before. I feel ********** and am moving to Telkom. Its a pity because I have been a loyal vodacom customer ever since I bought my first smart phone.</p>