Incorrect billing

I have a contract with a fixed billing of R99 per month with Autopage for a data line, the data line is not even being used but I'm billed up to R1500 per month, after numerous phone calls and emails to Autopage to investigate and come back to me since November, I do not receive any feedback just further debits per month, as well as a final demand. I've still continued to pay the fixed billing amount, but reversed all other debits. The contract with Autopage is obviously not worth the paper its written on as they are not adhering to it.