Federal number

says they are IRS and I will be arrested if I don't pay unpaid taxes!

There is a widespread IRS or "tax scam" being  practiced by scammers on both Canadian and USA residents.
USA tax payers should read this:
http://www.irs.gov/uac/Newsroom/IRS-Reiterate ... -Telephone-Scam

Short answer:
The IRS never initiates a contact via phone.
They always send a snailmail first.

Educational post:
This scam is also described as scam #11, here:
http://phonehelp.2truth.com/facts_rev.html

If you were contacted by an IRS impostor, you may file a report, after clicking on this link:
http://www.treasury.gov/tigta/contact_report_scam.shtml

I just got a call from this number, from a woman with a very thick Indian/eastern accent. Apparently I have to call back (I let the machine get it) before they issue a warrant for my arrest. Ha. I've always paid my taxes (yep, that's what this was about). Idiots - do they have nothing better to do with their time?

Got a message on my phone stating that she was from the Federal tax audit department and she received a legal application stating that my husband was committing tax fraud and that before they issued an arrest warrant for him he needed to call them back.  The woman had a very thick Indian/Eastern accent just like Alicia described above.