Contract cancelled a year ago - now I owe money

I cancelled my contract a year ago - Oct 2019 - because the 3G just stopped working in Aug 2019, and Telkom were unable to resolve - even after multiple sim swaps among other things. I requested that my account be refunded for the months that I could not use it.

Instead they have left the contract runninng and now I have a debt collector phoning me for R900 hat I owe Telkom.

I've logged, phoned, used the chat, posted on twitter - and the response is the same "The retentions dept will contact you and sort this out" Well guess what - another month has gone. Disgusting from Telkom to say the least