Telkom overbilling nightmare for incorrect contract by sales agent

I contracted to a 10 MB ADSL line with Telkom in March 2018 for R799 a month. All went smoothly for just under a year. In February 2019 a sales agent (the call was recorded but I did not note her name, unfortunately) contacted me and asked whether I wished to migrate to a fibre line which had been installed in my neighbourhood for the same price. I said no thank you. The next day she phoned me again and offered me a 10 MB line for R620 per month. Since it sounded good, I accepted the offer. The migration was done but the problem started when I received my first bill in March. She had put me onto the wrong contract. A Lie Uncapped 100 MBS line. I have been billed an additional R999 per month, in addition to other fees for over 5 months. I have spoken to numerous billing and sales agents to rectify the error, but to no avail. I feel like I am stuck in a black hole. I am supposed to be paying R620 a month all in and due to this error on the agent's part, my bill currently stands at over R3000 even through I have been payting in R800 a month. I have litreally been on the phone with seven different agents and logged 3-4 complaints and gone into a Telkom shop at Somerset Mall. I am at my wit's end as to how to have the billing error rectified. The billing department can't see my contract and the sales dept cannot see my billing record. When calls get dropped I cannot get back to the agent assisting me. I am beside myself as to how have this matter resolved. i have spent hours of my free time, my air time and paid in over and above what I am contractually obligated to pay. I was also told that the original sales call cannot be traced (the one that proves the agent's error). The agents I speak to tell me I am in the right, but no one amends my bill or refunds me. I have also been cut off twice and am currently cut off because I refuse to pay money I do not legally owe the company. Does anyone know senior personnel at telkom who can assist me?