Is this phone number a scam?

Complete scam. Harasser.

The caller said they were with the Department of Illegal Affairs and that they were settling an IRS matter.

Department of "Illegal" Affairs.  Right.  Who'd fall for something like that.

Caller left message saying he was from Illegal Affais at the US Dept of the Treasury. Said law enforcement was about to arrest me and that I should call back immediately. Wished me to have a "good life" if I didn't call back. When I called back and said I worked for a federal agency and asked for identification and other information, caller hung up and now I only get a busy signal when I call back. Obviously a fraudulet call, but disturbing.