Terrible service and harassment

In October 2019 I found out I'd be moving back from Cape Town to Jhb. I contacted Vox telecom to see if I can move my line, but they said there was no fibre in the area I was moving to , thus I cancelled the line. In November they said I had to pay for December as it's a months notice, not taking into account that I cancelled in October already... Proof was sent to them and they sent back apologies as they had a new system and it wasn't captured assuring me I wouldn't be held liable. Everything was good and then the bills started coming - each and every month with me sending proof and loosing my patients. THEN a debt company started phoning me on their behalf and threatening me with bad debt - let me repeat - I OWE THEM NOTHING! Then the calls started becoming weekly and quite honestly I can't deal with this anymore. What type of business are they trying to run??? Harassing customers who cancel on time and never missed a payment ??? I honestly don't know what to do anymore