Grossly incorrect Billing!

On the 1 Jan 2020 I logged a line cancellation request with Telkom (moved to Fibre) and the request was never completed; since then I've had numerous phone calls with support and billing department and logged multiple more cancellation requests (no one could tell me why it wasn't being cancelled) and the line was finally cancelled in June 2020. I'm now receiving an invoice for each month since I logged my cancellation which was supposed to be corrected to only be for January R200; but I'm being billed over R1000. I've tried calling the billing department again and again; the last time I waited 45 minutes and then in the middle of the call was cut off; and the consultant just never called me back (even though she had all my details). I'm at an absolute loss now on how to resolve this as I can't keep calling the billing department and waiting for 45 mins to an hour each time; only to not have the issue resolved.