No Feedback on R500 internetbill for 1 time usage

I have received my phone bill with an amount of OVER R500 for one internet call that lasted 2minutes. this over 1GB - My question is how it this possible? looking back at my cellphone history, I have never exceeded the amount of R1000 for my total bill. I lodged a dispute or an investigation and with great promise someone was suppose to phone me back to explain the usage, This did not happen and I have been phoning trying to sort this out however - i keep on getting empty promises ! the amount went off my account and I am sure Cell C just want me to forget about this. Extremely unhappy.