Are they asking you to call back on the same number? This is an AT&T wireless number. Usually these scammers are offshore and hiding behind voip services, which I expect is the case here and they're just spoofing this number.
Either way, report all calls to: https://www.treasury.gov/tigta/contact_report_scam.shtml
They are scammers. When questioned about the IRS lawsuit you get a different story ever time. They try to scare you into giving them money. When challenged they will hang up on you.