Go to the IRS.gov website under scams. There are two links provided for reporting this. I had 4 calls on my mobile from this number today. I called back & man hung up on me. An hour later, another message from them. I called back, let phone ring for five minutes. No answer. I have filed a complaint with the IRS and FTC.
Got two calls on my mobile from this number; person claims to be from IRS ('Internal Revenue Services' - note additional 's') saying this is my final notice and they're filing a lawsuit against me. (Dial the number back and one hears someone with a thick India accent - difficult to understand because of noisy background.)