If I saw something like IRS---Washington, DC on Caller ID, I would pick up and answer with "County Fraud Investigations Office".
Should the fraudster continue with their spiel, I would reiterate..."This is the county fraud investigations office, do you wish to report a fraudulent activity?"
My latest way to respond, if they ask for my wife or me by name is to calmly say "what is the color of the day and your operator number". When they don't respond (not that there is a good response), I tell them that they have called a non working number.