Overcharging & Useless

I signed up for a Telkom fibre line 3 years ago for the advertised price of R699pm. I've been billed R1702.81 every month since. I've paid R699 every month without fail for a year while still being billed R1702.81. For almost a year, after waiting 20 odd minutes for an agent, I keep getting told the fault is being escalated. After some time I stopped paying completely as I feel its not fair to keep paying for a issue that is not being resolved. Telkom has now handed me over and my bank has informed me that my credit rating is being highly affected. I have also sent the debt collecting company all the reference numbers and dates of all my call logs and still nothing. Telkom's customer service is truly the worse.