Do not give them any info. See my post and Elspeth's above for the links to the IRS and report this call. If the real IRS needed to contact you, they would send a letter.
And as far as not shutting them down, some of these criminals, extortion scammers and frauds are not in the US. They move around constantly even when they are and they spoof numbers & caller ID. It takes time to catch these guy's and then if they are in a foreign country, then you have more red tape. That country has to co-operate with ours and that doesn't always happen.
If the IRS wanted to talk to you, they'd send you a letter via good old snail mail. The IRS, for all it's faults, does not threaten people with legal action if they don't return a call. Sounds like a typical fake IRS scammer calling you. Report the caller to the real IRS at www.irs.gov.