incorrect billing

I have complained to Vodacom for the last 3 months but Vodacom simply refuses to reply. The incorrectly billed me for the last 3 months of last year, I started complaining the beginning of the year and since then my account seems to be in order but vodacom does not want to comment on this. I have phoned, left messages, email and been to the shops but still nothing. I do not know what to do anymore. Please help