The Treasury department says thousands of people have fallen victim to a phone scam, where a caller impersonates an IRS agent, and threatens criminal penalties and other legal action if money isn’t paid immediately.

The agency says it is the largest scam of its kind and has spanned more than a year and a half. Americans have been duped out of more than $14 million, and the calls have surged in recent months, authorities say.

“The clients that I’ve had inquiries from run the gamut,” says David McKelvey, an accountant at Friedman LLP, “an elderly person… younger people, they’re business owners, they’re employees.”

Paul Gevertzman, a tax attorney at Anchin, Block & Anchin, says the scammers have increased the sophistication of their deception, making the calls seem legitimate.

“They’re able to basically mimic an IRS address on your caller ID,” says Gevertzman, “It gives a little more validity. And you think, ‘this really is the IRS, because it says so on my phone.'”

The IRS says the agency will never make an initial contact with a taxpayer by phone. It will do so by certified mail.

If you have questions about your tax obligation, you can call the IRS at 1-800-829-1040.

And if you think you have been targeted by a phone scam, you can call the Inspector General for Tax Administration at the Department of Treasury to report the incident at 1-800-366-4484.