Not following through on telephonic conversations

I received a marketing call from Cell C in July/August 2019 to upgrade my existing contract. Instead of agreeing to upgrade I gave a request for cancellation and the person on the phone agreed to cancel the contract. I received another call at the beginning of October, again, to sell me an upgrade and I requested to have my contract cancelled. I asked if there was anything else I needed to do in order to ensure cancellation and was told the person on the phone would see to eveything and all I have to do was call on or after the 25th October to confirm that my contract was indeed cancelled (My contract ended on 24 October 2019). I called today, only to be told that I gave no notice and would therefore have to continue with the contract for an additional month! (I have an Unlimited contract, so an extra month costs me R1500!) This despite giving notice to 2 people on two separate occasion and asking if I needed to do anything further (at which point I should have been told to call the retensions department which I only found out yesterday)