Struggling to cancel old contract for multiple years

We have been trying to cancel the contract that we weren't even aware was still active for multiple years now. The number was last active more than 10 years ago.

Every time we call, we get told to send an email to some other address requesting cancellation, which we have done on multiple occasions.

A few months go by and debt collectors start calling again because the account is in arrears.

Why did the debit order stop in the first place as it wasn't cancelled from our side, so surely Vodacom cancelled it? Why was there no call or emails from Vodacom to try and resolve the issue before it was handed over to the debt collectors? Why is the contract not being cancelled as requested by email?

I am really tired of struggling just to get a contract cancelled, up to the point where I will never again in my life deal with Vodacom.