Incompetence in resolving disputes and *****ing client money

I made the biggest mistake of my life by entering into a 20GB wifi router deal for R399 per month. Now they charge me R823.68 (Excl. VAT) per month. Initially I phoned in to report the matter and they corrected the statements. But only for SIX MONTHS!! Since October last year I have had to phone in every month to get the matter resolved, but they only correct it for ONE month. And the last three months they did not correct it at all! When I phone their "Customer Service" (ridiculous that one should call it that) line they tell me the call was incorrectly "resolved" and they need to re-open the case. BUT IT NEVER GETS RESOLVED! Today when I phoned, the agent hung up on my call!! This is ridiculous and unexeptable!!!