Same experience. Got a voicemail saying the IRS was suing me and to call this number. I pretty much knew it was a scam because the IRS almost always uses the US Mail as their source of contact. However, I called the number and got a man with an Indian accent. He was asking me for personal information that the IRS would clearly already have, so I hung up.
SCAM!!!! These people called my 87 year old mother telling her that they were the IRS and that they filed a lawsuit against her for her taxes from 1998. These people are EVIL.