The IRS will not call you. They send letters via US Mail. The IRS won't ask you to pay in i-Tunes cards. The IRS will not wish you "good luck as the situation unfolds". Trust yourself and your instincts - do not let a bully intimidate you with threats that are nothing but hot air. This is a SCAM. Do not answer calls you don't know and never call them back.
Please report it: https://www.treasury.gov/tigta/contact_report_scam.shtml
Consider getting a call blocking device (or app for your cell phone)-they're available online and start around $50 or so. http://800notes.com/nb/search?q=call+blockers