If it was real, you would receive a letter in the mail or delivered by a court officer. The IRS does NOT call - they come without notice. You will probably get the usual "going to jail or the police are coming" message. It is a scam. That phone number is also probably a fake (spoof).
Called three times back to back and left a message. The message was from a Robbie White, who had a very think Indian accent and I could barely understand him. He said that he was calling from the IRS and they had a case against me. He said my full first and last name and told me to call him back immediately. He was stumbling over words and was incredibly hard to understand. Any time I have been called by a government agency or collections for any kind, they always leave an 800/888 number and an extension. I have never had someone say to call them back at a regular number. Sounds really fishy.