incorrect billing

I was surprised to see that I was billed double my vodacom account this month. When I enquired, I was informed that I had gone over my limit and was billed for another 3G's. The agreement was that I would be notified as soon as my data had run out and would not be billed. So when I had a warning that my data had been exhausted, I bought another 500MB. Which I could not activate until the beginning of the following month, despite the telephone agent assuring me I could activate it. When I phoned back to complain that I could not get on to the internet, the next telephone assistant was extremely unhelpful as one of my security questions I apparantly did not pass. Which was surprising because I passed all the security questions with the previous caller.