Vodacom wants me to pay for their mistake

Vodacom wants me to pay for their mistake. 4Aug2020 I called to cancel my contract Rosemary said she logged it, sep the contract was still active I called again, Nikhil said he can see it was logged but not submitted, he will log it again. Months later I called again I spoke to Luyanda and he apologised for the trouble, he advised that he has escalated the issue and it will be resolved within 24 to 72hours, I asked what will happen now because I decided to reverse the latest debit order, he said the arrears will be written off because the mistake was on their end. 4 days later it was still not cancelled, I called again I spoke to Nicollen and she apologised and advised the matter will resolved within 48 to 72 hours, and the arrears will be written off. Month later it was still not cancelled and I called again, I spoke to Sphelele and she said she can only get this cancelled end of Dec. 1st of Apr 2021 I get a letter of demand for the arrears of R609.95. Why is Vodacom bullying me to pay for their mistake, on top of that they are demaging my credit record. I have been trying to contact them but they keep referring me to Jared who was supposed to listen to my calls and advise, till today I have had any feedback.