Depriving my rights to cancel a contract by ignoring my calls, kidnapping me to their service which I longer need.

I had a contract with Vodacom which expired in February 2020, then I called in to cancel the contract in the 1st week of March 2020 where a consultant helped me and she confirmed that my contract was cancelled and gave me a reference number which I still have, and to my surprise l was called by Vodacom debt collectors saying my account is in arrears and I told them I have cancelled the contract in March 2020 and I was now advised to call the cancellation department where after numerous unanswered calls as usual I managed to speak to a gentleman called Latte or something like that but he dropped my call, I tried calling again but no answer as usual. Vodacom stop kidnapping me this is not a fair treatment to a customer. I have the right to cancel but you are ignoring my calls at the cancellation department. Vodacom resolve this matter before it takes us far.