The first quarter of this year has seen a 400% increase in phone fraud, according to the Telecommunications Industry Group (TIG), an association that numbers Telecom and Vodafone among its members.

Ten companies a week are having their PABX systems attacked by international hackers, who then make out-bound calls to premium pay numbers they've set up in North Africa or Europe.

The numbers often cost $US15 a minute, and the attacks often happen in the evening or weekends - so a business can be ripped off to the tune of thousands before it realises anything’s amiss.