MTN is billing me for a canceled contract and have ruined my credit record

I'm not happy at all. I took a contract with MTN through Mondo but canceled it within the 7days cooling off period that I was given.for some reason my contract was not canceled which let to MTN ruining my credit score. Mondo did all in their power to make sure the contract got canceled and they were assured by Mtn that its finally canceled. Today when I checked my emails there was an email sent on the 14 of June which is a bill from MTN. I don't understand why is Mtn still billing me for a contract that was canceled. I know called Mtn and the rude customer service called Lungile who refused to give me her Surname saying she doesn't owe me her Surname and she then hang up on me instead of helping me with my grievience. Im honestly tired of MTN this has been going since March we now in June and no one can help me since I speak to different customer services. Is it not ***** when you billed for something that you have canceled the contract for??