Here's tip #5 it's a scam: the call to me didn't begin with a personalized identifier like "This message is for Mr./Mrs. XXXX", as any legit collector would.
Just checked my VM, & a few days ago there was a message telling me to call this #. It was the first time they called, but said it was my 'final' warning. Funny, b/c I filed through H&R Block & was told any IRS contact would go through them first. Even more interesting is that most legit collectors will say, "This message is for Mr. XXX". Hopefully no one falls for this obvious scam.