1. It is illegal to threaten arrest.
2. I doubt they sent anything (mail fraud).
3. They are probably in India or Pakistan using magic Jack or VOIP to make calls.

They would want money wired to them or have you buy a money card at Wal-Mart, etc.  You would give them the card numbers which is as good as untraceable cash.

Yep, calling about criminal investigation filed against name and physical residence. They are also using the same stupid case number (CM100177) asking to call back at 917-965-8215. Started to read a 202 area code number to call back, but that's probably the old number that's on the Internet now. Idiots are too stupid to even chance the case number. How about being original you goof - just a little you mindless poop chunk. Nope - MORONS AT PLAY. But that's why they will be losers all their lives, making their [***] mothers and cowardly useless fathers proud of them. HaHaHa. - caller: Officer Jason Parker - HaHaHA!

http://www.irs.gov/uac/Tax-Scams-Consumer-Alerts
http://800notes.com/forum/ta-8c2f64bf2b91fa5/irs-warns-of-telephone-scam

Report the Calls to the real IRS here:
•report the incident to the Treasury Inspector General for Tax Administration at 800-366-4484.
•You can also file a complaint with the Federal Trade Commission at FTC.gov. Add "IRS Telephone Scam" to the comments in your complaint.

I got a call at 2 am from a person who said someone had the IP address of my computer and was currently using it to do bad things on the Internet.  The lady had a heavy East Indian accent.  I asked to talk to her manager.  He suggested that I give their technician access to my computer to fix the problem.

Initially they said they were with Microsoft.  I asked for their url and they gave www.techchurch.com, which is Symantec.

I told them I would not play this game and hung up.  They called back two hours later from a different 202 number, I didn't answer.