Vodacom didn't cancel my account when requested, now I have to pay for their mistake

Approximately 9 months ago I cancelled my Vodacom contract to move to a pay as you go plan, an unnecessarily lengthy process that actually took me two days to complete, but I did it. I was assured multiple times that I owed no more money on the account and that it would be cancelled with immediate effect.

Two months later I get a call telling me that I am two months behind on my payments, an obvious mistake, but I called the cancellations department and asked them what was happening, they told me I made no cancellation on my number. I was obviously livid, I ended the call, called again a few hours and another consultant told me I DID cancel the account, but that the cancellation order did not get processed...

What!? How do you create a cancellation order for a customer and not process it? How often does this happen? How many people don't confirm this? HOW IS IT OUR JOB TO MAKE SURE THAT VODACOM IS DOING THEIRS?!

I tell the consultant that I cannot pay the balance as a matter of principal. I made the effort to cancel my account and they didn't make the effort to process my cancellation order, and she told me that she completely understands and that I in fact will not owe anything on the account since this was an error on Vodacom's side. She was incredibly helpful and professional, and I was satisfied that the issue was resolved.

For the past 3 months or so I have been receiving calls from Vodacom's lawyers/debt collectors calling for that debt, which seems to be collecting a crazy amount of interest.

How is it that Vodacom's customers have to make sure that Vodacom are doing their jobs? Why do I receive these calls subtly threatening legal action, only to tell them that according to my call for cancellation, it was Vodacom that messed up and they said they would sort it out?

I am incredibly disappointed in Vodacom and this incident has completely tarnished what I had thought of as a competent service from them for the 20+ years that I've been a loyal customer.