Telkom and Debt In

Telkom changed our farm landline from a radio link" landline" to a cell phone based "landline" utilising an MTM sim card in about October/November 2019. We do not have MTN coverage in our residence and had to get to the nearest road to make a call. Nobody could call us. We had a Telkom Closer package and we paid for 3 months despite complaining that the service did not work. After paying in advance for 3 months of hopeless service we sent them a letter by email on 5 March 2020 stating that if they could not resolve our connectivity issue within the next month we would accept that they did not need us any longer as loyal clients of more than 10 years standing since 2006 and that they should terminate the contract. on 23 August 2020 having heard nothing further from Telkom and not having received any invoices since February 2020, I emailed them again after being informed by my bankers that I had an adverse listing because of an unsettled debt with Telkom. I received a reply on 9 September 2020 requesting the actual telephone number. I rep**** on the same day and sent them the requested phone number. I received a further reply on 16 September 2020 in which they apologised for the issue and informed me that the credits had been passed. I then received a Letter of Demand from Telkom dated 11 June 2020. Since December 2020 I have been harassed frequently on an almost weekly basis by a Company called Debt In who claim to be acting on behalf of Telkom. They have plagued me with phone calls and sms's making all kinds of threats against me for an outstanding debt. At first I was cooperative and on three occasions I submitted copies of the correspondence to different people at "Debt In" on behalf of Telkom. They have not let ups and are still harassing me. How do I get them to lay off and accept that I paid 4 months for a non existent service and asked them to cancel the service if they couldn't get it to work and they sent me a letter of apology and confirmed that they had passed the necessary credits ?