Incorrectly opened a new account iso Upgrade

I had a bad experience with MTN 2yrs ago, I was called for an upgrade and a 2nd account was opened, queried it and was sent from pillar to post. I never activated the new number and was double billed. I suggested that they move the new contract to my existing account and nobody listened. Now 2yrs later I am harassed by a 3rd party, Nudebt, on their behalf citing that I owe them +R18k. I need to settle this matter amicably by paying for the watch and phone that I received. Please assist.