Telkom ADSL problem

Telkom / Openserve shut down my ADSL line service (1/12/2020)

Based on this I contacted Telkom requesting them to cancel my ADSL line rental contract (followed the protocol advised on their web site)

The only response from Telkom was an invoice for Dec 2020 + an invoice for Jan 2021

Is a customer legally obliged to still cancel a contract if Telkom has shut down the service (without any written notification) - surely the contract is no longer valid if they stop the service and do not advise their customers in writing I have received nil written correspondence from Telkom (advising that my ADSL service would be shut down in Dec 2020

I bet if you do not pay their invoices Telkom will take legal action (and black list you) for non-payment

What are our legal options (how can they shut down the service but still send invoices and expect you to pay for the non-existent service

Can someone in the legal industry provide some comments on how to deal with with this situation