This is the voicemail that they leave.  "I am Officer Nicky Johnson from Internal Revenue Service and the hotline to my division is 415-690-2539.  Do not disregard this message and do return the call before we take legal action against you."

I called them back and they wanted a case number which they didn't leave.  They wanted my social security number.  I told them that I would not give them that information. The guy said "%u$k you" and hung up.