Account settlement

Good day

I keep receiving calls about an account which was handed over and settled in full last year with MBD, I'm now receiving a million and 1 calls from HP Harmond about a further amount you guys claim I owe. I have a settlement letter to show that the WHOLE amount was paid so where is this other amount coming from? Your collection department is just ******* cause really they don't serve a purpose as they couldn't even confirm what or where that amount is coming from. You guys making having an account with you a nightmare, you just get sent from pillar to post with no real answers.