An unintelligent scam. IRS won't call you....and a lawsuit entails at least a modicum of paperwork. There is no response to their phone number.  This is an ignorant attempt to get hard-earned money from you by this (these) cowards. Don't fall for it!!!!!

You forgot to mention that the IRS is an administrative agency.  As such it has many more options to collect taxes available to it than a lawsuit, which takes considerable time, effort, and cost to initiate.