Data struggles

I have been with vodacom for 10 years and recently decided to apply for a cellphone contract with them.My contract got activated on the 1st of September and on the 2nd of September I decided to convert R400 of my R500 airtime to data and I went on to buy data using *111# and chose the every month option meaning I'd get R400 of my air time converted into data every month (or so I thought).When I checked my balance I saw that I still had R500 worth of airtime plus the data I thought I bought with the airtime, so I called customer care. I was told that I wasn't supposed to choose the 'every month option ' and that the R400 for the data would be added to my bill.I don't inderstand why that should be the case because I will obviously have R500 worth of airtime every month and the R400 for the data can be deducted from there...I was told by the customer care agent to call ********** and there I spoke to Pinky (the apparent team leader) who did absolutely nothing to help and rudely dropped the phone while I was talking.