The telecoms regulator has warned businesses to be on the lookout for telephone system hackers.
It comes after a number of companies who had their systems breached were left with bills running to tens of thousands of euro for unauthorised calls.
The hackers have targeted organisations with their own telephone switch systems, known as PBXs, and reconfigured aspects of the switch to enable the making of outbound calls to destinations abroad.
The calls were often to premium rate numbers in those foreign countries, leaving the companies whose systems had been breached with bills of between €15,000 and €63,000, which they may have to pay.
According to ComReg, the hackers have breached the PBX systems by reconfiguring parts of the systems used for maintenance and voicemail.
The regulator said residential consumers are not at risk, as they do not have a switchboard or telephone switch system.
The issue arises due to inadequate security on the PBX systems, ComReg said, with the calls usually taking place outside office hours at weekends and over holiday periods.
As a result, the calls are not detected for some time.
Ahead of the Christmas holidays, ComReg said businesses should ensure appropriate security is in place on their PBX systems to prevent hacking.
It said strong passwords should be used for voicemail and maintenance ports and restrictions should be placed on access to premium rate numbers and overseas calls where possible.
ComReg also advised system administrators to monitor their telephony traffic on a regular basis to watch for unusual traffic.
Any business that is concerned about the issue is advised to contact its telecoms and PBX providers immediately, and to contact gardaí where fraud is suspected.