If you've been hung up on by a mystery international number, don't call back.

New Zealand is "flavour of the month" for robo-dialling scam calls, which may try to draw premium calling fees out of unsuspecting punters.

Telco companies are divided on the type of scam being employed, but are advising anyone who has returned a dropped call to check their phone bill.

2degrees spokesman Paul Brislen​ says a robo-calling piece of software is churning through the New Zealand phone book.

"That's why you get them in waves, and that's why suddenly New Zealand is the flavour of the month, because somebody has got to +64."

He says many are premium call scams, called the "wangiri scam" by the industry. Such scams aim to have a person return the hung-up call, routing them through the equivalent of a 0900 number and charging them an exorbitant fee.

"You call them back, and nothing happens and you're on the line for about 30 seconds and then you get a bill."

Paul is unaware how much people are being charged for the calls, but with 0900 calls "it could be anywhere from $1 a minute to $50 a minute".

The calls – from countries such as Bangladesh, Morocco, Canada, the United Kingdom and Cuba – might not be from the location indicated by the international calling code, as often internet-based phones are used to mask the true location.

"You could be calling from Madagascar and calling from a New Zealand number. It's hard to track the numbers as we can't tell where they're coming from, you can't block them and you can't tell how much people can get charged until it's too late."

People who have returned such calls should check their bill, and contact their phone company to contest the charge.

He says 2degrees does not have a blanket policy for such calls, and would talk to affected customers on a case by case basis.

Spark confirmed a surge in scam calls in recent months, but had not received reports on the wangiri scam specifically.

The hang-up calls were likely to be scammers using auto-dialling software to ring en masse; the silence due to either the dialler failing to pick up "human activity", or the caller failing to reach the connection in time, Spark communications advisor Cassie Arauzo​ says.

Often these callers purported to call from a recognisable company or have investment opportunities, and return calls accrued international calling charges.

Wangiri scams tended to hang up after a few rings, generating a missed call to be returned, she says.

"That doesn't mean wangiri calls are definitely not happening – we can't see it on our end. But if customers are getting charged premium rates we do encourage them to report it to us," says Cassie.

Increasing sophistication of scammers meant customers need to be vigilant, she said. Spark is developing an education campaign on scams currently, with a webpage for reported scams due to go live in June.

Industry group Telecommunications Forum is doing the same, but declined to discuss details. spokeswoman Sarah Brown says they are working with telcos to proactively combat scam callers.

"The telcos are in a position to act once they know that this activity is going on, but it's difficult to predict," says Sarah.

-Stuff/Thomas March