Just called, I ignored it, but left no message.  I did a google search and found this on another website:

Yes the number called me and it was an automatic recording informing me that the IRS has filed a lawsuit against me and to return the call. I called back and received a busy signal. As an attorney, I'm fairly certain that prior to any legal proceeding with the IRS, a taxpayer would receive numberous formal, written communications. Now, I'm concerned about what information was retrieved using my phone number.. and for what purpose.