I got a phone message in which a robotic voice identified itself as the Treasury Department, contacting me about some matter related to my taxes, and saying if I didn't reply I could be arrested. Is there any chance at all that's legitimate?
Chances are almost exactly zero that this is a legitimate call. However, if you would like to verify that with the Department of the Treasury, they have a fraud hotline at 1-800-359-3898, which you can find on their contact us page.
Assume any information they give you is false, and only contact the Department of the Treasury (or any other agency/company) though numbers listed on their website. If the scammers continue to harrass you, you can report them to your local police.