Bad customer service

I opened an account with them for data subscription, the contract stated that I was supposed to pay R399 per month for 40 gigs data. This year around March 2018 I got an invoice stating that the amount due was R2900. I tried to dispute this, made several calls and no one bothered to resolve it. When I went to one of their branch in Cavendish Mall in Claremont, Cape Town, one of the sales people confirmed that an error was made at Head Office so they cant assist from their side. As of today the current amount on the invoice is more than R7000. They cancelled the services beginning of April and handed over my account to their legal team. May you kindly intervene. I will not pay such an amount, Someone has to account for such an error. I believe you can help. Regards Lwazi