CONTINUOS LOSS OF DATA

How do you explain Vodacom...? I get 2GB data every month that I use on my Wi-Fi - for evening and weekend use. The no is ********** . Last month when I received it on the 1st, it was depleted before 17.00 that same day. I queried it and apparently it was all correct - the data was used. On the 13th of last month, I bought 1GB of data on Cell C - valid for a month. Yesterday when the data expired, I still had 450MB of data left. I switched on my Wi-Fi which I have again my new months' 2GB of data. When I checked this morning, I had 189.63MB left. How do you explain it? It is the same phone, the same usage, no apps open (otherwise Cell C data will also get used). This just proofs that the data charges on my account IO40213-7, number ********** is also incorrect as I do not use the data!!!!! I have Wi-Fi at work and work 7 days a week.