In the middle of the night, the calls come in waves, going silent after a ring or two. It’s no one you know, just someone who really wants you to call back. That’s the scam.

The Federal Communications Commission is warning consumers about robocallers who ring up targets, then abruptly hang up. Those who call back end up racking up charges that are mostly paid out to the scammers. It’s known as a “Wangiri” scam, Japanese for one-ring-and-cut.

“These calls are likely trying to prompt consumers to call the number back, often resulting in per minute toll charges similar to a 900 number,” the FCC said in an alert Friday. “Consumers should not call these numbers back.”

To the unsuspecting person, the one-ring calls appear to be from someone in the United States, because the phone numbers begin with three digits that resemble American area codes. But the FCC says the scammers are using international numbers whose country codes also begin with three digits, making it easier to trick people.

The agency said the callers are using the country code of Mauritania, which shows up on phone screens as 222, and Sierra Leone, which is 232. The robocallers appear to be concentrating their efforts in New York and Arizona.

Scammers also can trick people into answering their phones with a technique known as neighborhood spoofing, in which they manipulate caller ID information to mask their real phone number and appear to have been placed locally.

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In addition to spoofing the country code and the seemingly urgent timing of the late-night phone call, scammers also are leaving voice mails, urging their targets to call back to “collect a prize” or to notify them about a “sick” family member, the FCC said.

To avoid becoming a victim, regulators say, don’t answer or return calls from an unfamiliar number and check whether an area code is international before calling a new number. Consumers also have the option of asking their phone company to block outgoing international calls. “Always be cautious, even if a number appears authentic,” the FCC said.

An effort is underway in the U.S. Senate to crack down on robocalls, which pinged consumers’ mobile phones roughly 26 billion times last year, according to one industry estimate. The bipartisan legislation would give the federal government more power to hit those who engage in spoofing with bigger fines. And officials could press phone carriers such as AT&T and Verizon to give their customers better tools to detect when incoming calls are from spammers.

Those who fall victim to the one-ring scammers and get billed can ask their phone providers to wipe away the fee, the agency said. If that doesn’t work, they can file a complaint with the FCC.