Telkom overcharging for service

The incredibly disappointing service rendered by Telkom will force me to cancel my internet service with them as I can not afford it anymore, since they started incorrectly overcharging me and after 3 months they still close my case numbers as being resolved without passing my credits, or rectifying my account so that the overcharge will not occur anymore. I do not know where to turn to or whom to ask for help as everyone seems to pass the buck to another person to attend to it and in the end nothing gets resolved. I am told that I have to settle the full account every month regardless of it being incorrect else my service will be suspended. How is it legal to keep on taking more money from a customers account than what is owed for a service rendered? I have a Home Uncapped (20 Mbps) Fibre line. Subscription is R1007.76 per month all icl. Telkom all of a sudden charges me R1701.80?