You’ve been waiting for a call from your doctor all day. Finally, you get a call from a number with the same area code as yours, and you pick up. Instead of your doctor, a robotic voice greets you, informing you that you’re in trouble with the IRS and need to call back immediately to avoid legal penalties.

What do you do? (a.) Do what you’re told and pay, (b.) hang up and call the IRS or (c.) hang up and call your doctor to get an update on your health. Correct answer is “c,” because the IRS will never call you to demand repayment.

That might be an easy answer.

But many robocalls prey on our most generous nature. While some robocalls scare consumers, others attempt to convince them to donate to fake charities, and sadly, many fall for their tactics. Consumers reported losing $905 million to fraud in 2017, up $63 million from the previous year, the Federal Trade Commission said in a recent statement.

The problem isn’t going away any time soon. Last month, 1,591 robocalls were placed every second, amounting to 137.5 million calls every day of the month, according to a report published by YouMail, which provides a free robocall-blocking service. Not only has that rate consistently increased over the course of the past four months, reaching a record high this past month, but it has also increased by 63% since June 2017.

Watch out for charities using well-known voices

No, Barack Obama isn’t calling you, to ask for your money or, probably, anything else. Sometimes fraudsters use robocalls with famous people’s actual voices, claiming that they’re raising money for various causes. The Connecticut Better Business Bureau issued a warning in 2017 about robocalls that lured consumers with famous voices and then prompted them to press a button to talk to a “donation agent.” If you get one of those, just hang up.

To fight back against robocallers, apps like Truecaller and YouMail tricks the caller into thinking that the number they are trying to reach is out of service, which deters future robocalls. Currently, YouMail has 10 million registered email users and has processed 8 billion robocalls, said Alex Quilici, chief executive of Irvine, Calif.–based YouMail, which produces a robocall-blocking app.

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Scammers take names similar to legitimate charities

Fraudulent groups with legitimate-sounding names like Help the Vets and Veterans of America placed more than a million robocalls that resulted in people donating over $435,000 in cars, boats and even real estate.

The groups, which also operated under the name Act of Valor and Saving our Soldiers, were started by Travis Deloy Peterson in 2012, the FTC alleged. What’s worse, the organizations’ names sound similar to legitimate charities such as Vietnam Veterans of America and Disabled American Veterans. You can check the charity’s rating at CharityWatch.org.

Earlier this month, the FTC filed a case against Peterson that alleged his businesses were not recognized as a charitable organization with tax-exempt status. Peterson could not be reached for comment. To prevent other fake charities from taking advantage of consumers’ goodwill, the FTC announced Operation Donate with Honor, an initiative to help consumers identify legitimate charities that assist members of the armed forces.

See also: 2017 was the worst year yet for robocalls

Screen shot of Truecaller’s spam-detection function. Truecaller

Millennials are more likely to respond to robocalls

“Scammers can reach millions of users a day, and so they don’t need to make any demographic a target,” said Nick Larsson, head of growth and partnerships at Truecaller, a free spam blocking app which reported 100 million daily active users.

In fact, one in every 10 U.S. adults received a spam call, and on average has lost $357 as a result, according to an April 2018 Truecaller report. Of the 2,036 people surveyed in the report, 24% were men aged 18 to 34.

Along with the elderly, millennials are much more likely to respond to a robocall since they don’t get calls often, according to Ethan Garr, vice president of product at RoboKiller, a robocaller blocking app that engages in humorous conversations with spammers using pre-recorded answer bots.

Also see:Getting too many spam phone calls? Here’s why

Screenshot from robocaller-blocking app RoboKiller. RoboKiller

Not all charities that use robocalls are fake

Not all charity robocallers are fake, said Larry Lieberman, chief operating officer of Charity Navigator, a nonprofit organization that evaluates charities to help donors make informed choices.

Legitimate nonprofit organizations like the Environmental Defense Fund, Foundation Fighting Blindness, Mothers Against Drunk Driving (MADD), Operation Smile and the Special Olympics have all used robocalls in their fundraising.

MADD’s acting CEO, Vicki Knox, said in a statement: “MADD is always trying to find cost efficient means of communicating with its donors and past supporters, including the use of automated calls. Whether it’s a reminder about an upcoming event or a simple thank you, we’ve found that it can be a great way to keep in touch with our constituents.”

(None of the other four nonprofits responded to requests for comment.)

Additionally, the American Red Cross uses robocalls for “a variety of purposes, including reminders about appointments and information about upcoming blood drives,” a Red Cross spokeswoman said in a statement. But Lieberman added, “We do not encourage any donor to give based on a robocall.”

“A donation is an investment in your community,” he said. “If someone called you up on the phone and said, ‘Here’s great investment to make for your retirement account,’ you wouldn’t give them your money without checking out.”

Charity Navigator flags charities for suspicious or illegal activity, as well as enabling a search for ones that are most effective in using donations for their stated purposes.