Refusal to cancel my expired contract

I had 2 contracts with MTN with the following numbers:

*** ***

Both contracts were due to expire/mature at the end of September. I made arrangements for cancellation, and I was told that it went through. I also paid whatever outstanding amount there was.

I am currently having a problem with the *** number. I got an invoice that I am owing R550 last month and another one this month. Upon calling MTN agents I was informed that the number wasn't cancelled and 40 Gigs were loaded on it.

I have proof of cancellation. The first day I cancelled the number was the 27th of September 2021. I wrote the reference somewhere and I lost it. I then did a follow up a few days later and was informed that the cancellation didn't go through. I then called the customer service on the 7th of October to affirm the cancellation of both numbers and was given the following reference numbers: *** and ***. I even received an SMS stating that I'm now on prepaid and MTN is giving me R10 airtime.

It is unfair for me to be billed when I clearly stated that I am cancelling the contract, on time and was billed the outstanding amount, which I paid.