Occasionally he would get a new "agent," with whom he would restart the prank all over. He said he also used the name Ryan Reynolds for a while, with the scammers apparently not picking up on the joke.
Eventually, he says the scammers got so fed up with him that someone snatched the phone and identified himself as a manager.
"I'm like, 'Stop hanging up on me. Tell me what's going on.' And he goes, 'No. You stop pranking me,'" Underhill said.
He said he broke out laughing and pointed out how strange it was that "I have a criminal telling me to please leave them alone."
Underhill said he spoke to eight or nine people in total. Although the scammers used a phone number with a 613 area code, which would place them in Ottawa, he's pretty sure their true location was outside the country — and that it was a large operation.
Underhill said that during one of his many calls, the person on the other end of the line held up the phone to let him hear the din of all the other "agents" making calls.
"You could hear the amount of people in the room talking at a huge call centre. It wasn't three or four people in a boiler room. It was probably 40 or 50 people," he said. "It's a big organization."
Ultimately, Underhill said he hopes his prank tied up the scammers' phone lines for a bit, preventing them from reaching someone more vulnerable. He also wants to get the word out that such calls are not to be trusted.
The Canada Revenue Agency told BuzzFeed Canada that people should contact the agency directly through its 1-800 number or through its official website.
"The CRA never asks taxpayers to provide their personal or financial information by email, text message, or by clicking on a link," a spokesperson said in an emailed statement. "It also never asks for payments by a prepaid credit card or a gift card, and does not inform taxpayers by email that they have tax refunds. The CRA never threatens taxpayers with police arrest or jail time."