Worst Contract from Telkom

I paid up my contract account with Telkom based on the last invoice I received from them on 01 Feb 2023, in March I received an invoice stating R0.00 which was the last invoice I received from Telkom, next thing I get calls from a debt collector saying I still owe Telkom. I called their line 081180 to understand how and why, which I spoke to the billing department to tell them the situation and they transferred me to the cancellation department which told me it's not their departments duty to solve such queries. Now that leaves me baffled and without any direction as to whom I should solve this issue with, both departments were avoiding escalating the matter...