Standard IRS Scam stated I owed money and to call him back, could face grand jury action and arrest. No legitimate agency will contact you via phone to collect money. If you truly owe money the will send the notice via mail. Do not call the scammer back as this will only verify your number as valid. You can file a complaint at https://www.treasury.gov/tigta/contact_report_scam.shtml
Threat that if I don't call back, I will have to appear before a magistrate or face criminal charges and possible appearance before a grand jury. Claim to be representing the US Treasury.