Comreg (the Commission for Communications Regulation) said it had received complaints from members of the public in recent days after customers began receiving phone calls, usually with a +23 9 prefix.
Often it appears there is someone else on the other end of the line. If the person receiving the call is not a bill-pay customer and they ring back the number, their credit can be stolen.
The calls appear to come from obscure locations, including the small independent island of São Tomé and Príncipe off the west coast of Africa.
More than five years ago, when a similar scam was in operation affecting Irish phone customers, ComReg temporarily blocked calls involving São Tomé, as well as other locations including the Cook Islands, Tokelau, and Diego Garcia.
In some of the recent cases the number from which the calls originate appears to be in Turkey.
Customers using various mobile phone providers have been affected, although Comreg said it did not know how many people had been affected.
Tom Butler of Comreg said these calls may not be genuine and it was investigating the situation under Section 23 (2) of the Universal Service Regulations.
“We are advising consumers not to return missed calls to phone numbers that begin with these numbers, or to answer calls from such numbers, particularly if they do not recognise the number calling or they are not expecting a call, as they may incur significant charges.”
He said moves to combat the scam could include blocking, on a case-by-case basis, access to numbers or services where fraud or misuse may be occurring.
A spokesman for O2 said he was unaware of customers on the network being affected by this latest scam, although some customers did fall victim to a “smishing” (texting scam) earlier this year.
The company was forced to issue a warning to customers about the scam, in which mobile phone users received fraudulent text messages attempting to entice them into releasing personal details.
In the February scam the text messages purported to be from O2.
Vodafone also recently warned customers to be cautious about unexpected emails claiming to be from the company or sent on its behalf.