If the estate went through probate (assuming all protocols were followed) and you didn't cosign then you shouldn't be on the hook for anything. I would politely tell them the estate is closed and if they have further questions they can contact the court. The court will have all your filings for notifications and such, so they'll be able to see they missed out on being able to collect. There's a limited window for creditors to come collect money for this very reason.