Bill is being overcharged

I am very irritated with the service that I am receiving from telkom because I was changed my spending limit and for the first two month after I changed my spending limit my bill was fine, then from August they overcharged me and even in the September bill they did the same thing when I call customer service they not helping me they telling me that my spending limit on their side is R30.00 and they don't know why I am being charge above that spending limit.