Failure to cancel a contract after i paid settlement fee

What a horrible experience i had with MTN. I canceled my contract last year October i got even called MTN after a few days to make sure the contract is canceled.

I paid the money they requested for cancelation in October 2022. Last Friday 26 May 2023 i get a call from a company saying they are a collection agency i owe MTN and i was handed over.

The disgust i having been getting since Friday 26 May have have been transferred from Pilar to post by *********** consultants.

Seems like none of the consultants know how to hlp me. I have further annoyed by this because it might affect my credit score.