RS does not file lawsuits. Tax disputes are civil matters. 90% of all IRS tax disputes are settled administratively. IRS has absolutely no authority to call on local law enforcement for anything. Local law is never involved in IRS tax disputes. If LE is necessary, US Marshals do the job. IRS does not cold call anyone and all contacts are initiated by US mail.
Same thing happened to me today. Message left saying they had been trying to reach me and this was their final attempt, that the IRS had a lawsuit against me and to call the above # to get more information on the file!!! Total scam.
While this appears to be a nasty scam, it's not as nasty as the actual IRS which will do violence to you if you don't give them money. The reason this can even happen is because there is a nasty group like the IRS out there. They are scary and this group is just pretending to be them.