These calls are routed through the internet to give you a false and usually non-existent caller ID# It does no good to call them back or report them to the phone Co or authorities.
Very interesting note however. The IRS calls stopped about four months or so ago, right after I saw a story on the news where they busted a big call center somewhere that was doing this scam. I used to get them about once a month.
I love jacking with telemarketers, I used to screw with the guy's selling aluminum siding back forty years ago.