Message was text-to-speech (computer generated) and said the following:
"This call is a official final notice call from IRS. The reason of the call is to inform you that IRS has filed a lawsuit against you. To know more information call immediately our IRS department number 520-353-4723. I repeat 520-353-4723."
Reminder: The IRS uses the USPS to inform people of lawsuits, not the telephone. Even if the IRS did contact people by phone, the certainly wouldn't leave a voice message without specifying who the message was for or giving some sort of reference number.
I have to believe this was generated by a foreigner due to the slight irregularities in wording and the use of computer-generated text-to-speech (probably to hide an accent or poor English.)
Finally, the phone number they said to call back is provided by an Arizona VOIP carrier. (BROADVOX-CLEC, LLC - AZ)
As a VOIP, it can be used by anybody anywhere in the world.