Telkom contract cancelled but handed over to debt collectors

I signed a 2 year Data Contract with Telkom.

The service never worked and no one would come out to fix it, and penalties to cancel was enormous. I never got 1 MB. But for 2 years I waited it out until April 2019 when it expired.

Telkom phoned me to notify that my contract is expiring and asked if I would like to extend. I said NO. They continued to deduct money monthly, which I reversed each time, assuming their admin is slow to catch up.

Today I got a call from a debit collecting company (DEBT-IN) threatening to effect my credit score at ITC negatively if I dont pay 4 months plus interest and penalties. Apparently its Telkom policy that I need to send them a letter to cancel - after they have a recorded conversation where I explicit said they should cancel my contract. How can they get away with this? This bullying, taking money and threatening consumers. Why are they trying to catch people out? Can someone from Telkom please assist me.

I do not understand.