People in Australia are falling victim to the 'one ring and cut' scam, with migrants said to be more at risk.
Australian phones are being "flooded" with so-called wangiri fraud calls, according to the Australian Competition and Consumer Commission (ACCC).
Named after a Japanese word which loosely translates to "one ring and cut", wangiri calls involve users receiving a mysterious missed call from overseas numbers.
ACCC deputy chief Delia Rickard said her organisation had seen a dramatic increase in scam phone calls from international numbers.
"Australian phones are being flooded at the moment, with what we call wangiri scams. And this is where you get a call from an overseas number, they ring once, hang up and what they want you to do is to call them back."
If the call is returned, the scammers try to keep people on the phone for as long as possible.
Technology expert Valens Quinn said they might play music, read a horoscope, or simply put people on hold.
"Well it's just like a 1300 number type of thing, so you get charged for the call. The longer the call is, the more you get charged. The scammers can basically set the cost per minute."
The ACCC has received 190 complaints about this kind of fraud since January. But they said people don't always report it, so the actual numbers could be much higher.
The ACCC's Delia Rickard said migrants are more likely to get scammed.
"If you know people in the country that they're calling from, particularly if it's your home country and you've got family there, you're definitely going to be more at risk because you're going to look at that and hope that it's someone you're waiting to hear from and call back."
"So I do think that people of CALD backgrounds are more at risk of this, just because they are likely to have more overseas connections."
In an article on their website, Telstra warns customers against answering unknown international numbers, and suggests blocking unwanted numbers.
Optus encourages customers to report incidents to the ACCC's Scamwatch.
And while Vodafone said they actively block known scam numbers, they said there are limits to how much phone companies can do.
Vodaphone said "monitoring and blocking is ongoing because unfortunately as soon as one number is blocked, the fraudsters can switch to a different number".
But advice for consumers is pretty simple, said Valens Quinn.
"Well it happened to me the other week actually. I got a call from Cuba. I know I don't know anyone in Cuba, so I didn't answer it - which is what you should do, don't answer the phone."
"Really it just depends on whether you know someone in that country. Most likely you probably don't, so just don't answer it."