Vodacom charges for 1GB data but allocates 500mb

I upgraded my phone about 2 weeks ago and then added 1gb data to the account. 24 hours later I got sms notification of 500mb added. After phoning 5 times, I was told that I was billed for 1gb, 500 mb was issued, and used, whilst the phone's data was set on CellC and Wi-Fi whilst being at work. I picked up the discrepancy as I checked the phone app, and the internet website, which differ not only from each-other, but also from what Vodacom's staff can see on their side. Which is true? Why can't your system allocate and bill correctly? What should we believe is the correct information: 114, Phone app or website? I need my account sorted, my data and account fixed, and similar ********** actions by Vodacom eliminated. How can I know my account won't be recklessly treated or **********ly abused? Please assist! I need answers on all the above questions. Please, answers not lame excuses.