Just ABSOLUTELY ignore those calls. And never, EVER talk with anyone about any supposed past due debt. And especially, NEVER agree to pay them a dime. That will create a problem worse than any worrisome call you may receive. Don't even try returning their calls. Just be comforted in the knowledge that you have been able to take care of your debts the last few years. You have NOTHING to be concerned with. Good luck!
This is a collection agency which specializes in "Zombie" Debt. They will use various questionable tactics to try and re-set the timeline of the Statute of limitations". Know you credit report and use that knowledge to stop the questions designed to create a debt that died off long ago.