Your phone rings once and the caller leaves no message. Thinking you missed a call, you call the number back.

Someone answers or maybe you get a recording. You stay on the phone listening to the person or trying to decipher what the recording is saying.

Unknown to you, though, you have been connected to a line with high interconnect fees, similar to calling a 900 number, and a bloated phone bill is on its way.

The Federal Communications Commission warned consumers about this “one ring” scheme in an alert last week. It’s also 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,” the F.C.C. said. “Consumers should not call these numbers back.”

The commission said the calls were relying on the 222 country code of the West African nation of Mauritania, and were reported to be widespread in New York State and Arizona.

In April, Americans received 4.9 billion robocalls, or 163 million calls per day, according to YouMail, a robocall blocking service.

It’s no wonder: Robocallers have become more advanced in how they hide their numbers and spoof — or imitate — numbers from other places.

Defenses against robocalls have also expanded, with the development of apps to block them and phone companies investing in technology allowing customers to identify such calls.

But what happens when the call’s purpose is to bilk consumers?

“International revenue share fraud is when fraudsters intentionally drive phone calls to premium rate phone numbers,” said Tim Prugar, the vice president of operations at Next Caller, a real-time verification technology company. “These fraudsters get a kickback from the call.”

The potential cost of one of these calls to a consumer “varies widely based on the location of the number and the amount of time spent on the phone,” he said.

Although the calls are being trafficked from overseas, the phone number that is spoofed — the one seen on a caller ID — takes the format of a phone number from the United States.

“Advances in technology allow massive amounts of calls to be made cheaply and easily,” the F.C.C. said. “In addition, spoofing tools make it easy for scammers to mask their identity.”

The agency recommended not calling back unrecognized numbers, especially those that appear to be from overseas. It also suggested blocking international calls and checking phone bills for unidentifiable charges.

“They put 100 million calls out and they know 1 percent converts into sales,” Mr. Prugar said, referring to those behind the robocalls. “Just by sending those phone calls, they are going to get one million people, at least.”

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