Robo call scam that begins its recorded outgoing message the instant the phone is answered, or goes to voice mail.  As a result, I could only hear the last half of the message which referred to "this case file", and demanded a call back.

This is apparently the well known IRS (you owe back taxes) scam, or the "we're filing a complaint against you" scam.  Either way, or whatever, it's an attempted SCAM.  If you fall for it, they will demand you send them money via prepaid debit cards.

There are numerous telephone and email scams in which individuals claim to be employees of the Treasury Department. These scammers often state that they are from the “Department of Legal Affairs,” offer grant money in exchange for you wiring a small payment, or threaten to arrest you within a short period of time unless payment is made. Do not provide personal information or payment to these individuals.  Their actions are crimes under Titles 18 and 31 of the United States Code, and the Treasury Office of Inspector General is working to stop them.

If they claim to be from Treasury, report  it to OIGCounsel@oig.treas.gov with a description.  If it was a phone contact, include:
the exact date and time that you received the call(s)
the phone number of the caller
the geographic location and time zone where you received the call
a description of the communication.

If they claim to be from the Internal Revenue Service (IRS) or it is about taxes, report it to the Treasury Inspector General for Tax Administration at:
https://www.treasury.gov/tigta/contact_report_scam.shtml

https://www.treasury.gov/services/report-fwa/Pages/Report-Email-Scams.aspx