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.

Two Degrees has warned that some office phone systems and contact database software will not permit their users to dial numbers using its 022 prefix.

Under number portability regulations, customers switching to Two Degrees will have the option to adopt one of the third mobile operator's 022 numbers, or bring their 021 number from Vodafone, or their 027 number from Telecom.