THIEVES are hacking into company phone systems and making calls for free, costing industry hundreds of thousands of euro.
One firm has already had calls worth more than €250,000 made on its system this way.
In another recent case, calls worth €90,000 were made without the knowledge of the customer.
Communications regulator Comreg has warned that more could be hit when businesses close for Christmas.
Twelve cases have been reported to Comreg in the past four months.
The regulator said that any business or organisation, including schools and charities, with a PBX exchange to allow for multiple phone sets could be affected.
A Comreg spokesman said: "In the festive season, many businesses will be closed and may not notice their phones making thousands of international calls automatically."
In the case earlier this year where calls worth more than €250,000 were made illegally, Comreg was able to intervene, but the firm was still left with a bill for €100,000.
The problem with PBX business phones was that they often had access ports that allowed maintenance firms to dial in remotely to diagnose problems.
However, these often have weak or default passwords such as 1234 which are easily exploited by hackers. Comreg urged businesses to consider disabling remote access.
Maintenance firms should also make sure they had strong passwords.
Companies should consider getting calls to premium rate numbers barred if they are not needed, while access to international calls could be better controlled.