If you have been receiving strange calls from unknown overseas numbers you are not alone.

New Zealand has become a favourite destination for robo-dialling scam calls, and people are being warned not to call back the mystery international numbers.

If you do mistakenly call back it could cost you plenty, with phone companies charging premium calling fees from unsuspecting people.

Kiwis have reported receiving calls from countries like Cuba, Morocco, Tunisia and Bangladesh, to name a few, in the last few weeks.

Stuff reports that local 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 said a robo-calling piece of software was 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."

These calls are referred to as the "wangiri scam" by the industry.

The idea is that a person returns 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."

Experts agree that the best thing to do is nothing - do not return the call from places and people you do not know.

- NZ Newswire