There was once a time when picking up a call from an unfamiliar number didn’t guarantee you’d be talking to a robot. For several years following its introduction in 2003, the Do Not Call list successfully sheltered individuals from unwanted calls about gym memberships and cheap travel packages. Companies respected the list, and if they didn’t, they faced legal ramifications.

Then, at the turn of the decade, something changed. Telemarketing scams began trickling through the cracks, and today the Do Not Call list is about as effective as a free cruise offered over the phone is free.

So what happened? It may not be evident to current members, but the National Do Not Call Registry does work—with some numbers, at least. According to the Do-Not-Call Implementation Act, telemarketers (excluding surveyors, politicians, and charities) can be fined up to $40,000 for ignoring the list. So when it comes to calls from legitimate, law-abiding businesses within the U.S., the Do Not Call list is a useful barrier. Problems arise when callers never intend to follow the law in the first place.

Around 2010, the same time the internet made international calls a lot less expensive, phone scammers began relocating outside the U.S. Whether they’re calling from India or Jamaica, voice over internet protocol technology makes spamming numbers with prerecorded messages cheap and easy. Another trick, known as "call spoofing," allows frauds to input fake caller IDs to make it seem like they’re calling from within the country. Some telemarketers even go so far as to call from the recipient’s home area code, leading the person on the receiving end to think it’s someone they know.

“It’s difficult to identify who’s actually placing the call because of the call spoofing,” Maureen Mahoney, a public policy fellow for Consumers Union’s End Robocalls campaign, tells Mental Floss. “So that also makes it difficult to track these people down.” Even when authorities do catch up to operations working in foreign countries, most of the money scammed from consumers has already been spent. It’s no wonder that the U.S. loses billions of dollars to scam calls each year.

Phone owners are well aware of the problem. “We actually sent out an email to our list asking what’s one of the issues you’re most concerned about, and overwhelmingly the response was robocalls,” Mahoney says. Consumers Union took action by launching their campaign to end robocalls in 2015. Instead of going after lawmakers, who often receive the brunt of the public’s blame, the initiative targets phone companies. A petition on the organization's website calls on phone company CEOs to “provide free tools to block unwanted robocalls before they reach my phone.”

“We really believe that the phone companies are in the best position to address the problem,” Mahoney says. “They’re the ones with the best engineers and the technology and the know-how.” Some industry leaders have taken steps to tackle the issue. Time Warner customers have the option to sign up for Nomorobo, a service that blocks illegal robocalls, for free. AT&T made a similar option available for select devices in December, and T-Mobile rolled out a robocall-blocking feature of its own in April. But there are still many companies that have no such resources available, or only offer them at an additional cost.

If electing to block robocalls through your service provider is impossible or impractical for you, there are other ways to protect yourself. When an unknown number lights up your screen, don’t pick up. Sometimes a “hello?” is all the information telemarketers need to confirm you’re a living human being who is worth calling again. If you do decide to answer, don’t be afraid to hang up as soon as things start feeling fishy. Staying on the phone gives scammers more opportunities to squeeze information from you, so even asking to be taken off their list is more trouble than it’s worth.

One of the most notorious scams to look out for today is the IRS phone scam. To trick their victims, callers (sometimes calling from a bogus Washington D.C. area code) will say they work for the IRS and demand to be paid immediately. Americans have been cheated out of tens of millions of dollars as a result of this scheme.

Mahoney also warns consumers to be wary of calls claiming to come from card services (“We can offer you a lower interest rate!”), tech support (“We can fix your computer!”), and even your phone company (“You need to pay your bills! Could we have your card information?”). Even if you suspect the call's legitimate, it's always best to end the conversation and call back using a number you trust. “If someone’s asking for your personal or financial information, hang up the phone right away,” Mahoney says. “Report it to the FCC.” You can contact the Federal Communications Commission with your complaints here.

We already know that the Do Not Call Registry alone isn’t enough to keep telemarketers at bay, but it doesn’t hurt to keep your name on the list—even if all it does is cut down the amount of robo-harassment you receive each week by a call or two.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.