O2 Ireland sent text messages to 70,000 customers yesterday apologising for overcharging them for mobile calls received while they travelled abroad.
The State's second-largest mobile telephone company, which has 1.3 million subscribers in the Republic. has pledged to repay all its customers affected by the overcharging before the end of July.
In a statement responding to an article published in The Irish Times, O2 confirmed that 70,000 contract customers had been billed twice for calls received while travelling abroad.
The problem did not affect any of O2's one million prepaid customers who made calls abroad.
The double-charging applied to customers who received calls while travelling in Europe in the period since late March. The problem occurred due to a billing system error and was not disclosed to customers until it was reported in the press on Friday.
Ms Danuta Gray, O2's chief executive, said that customers had been double-charged for incoming roaming calls made while travelling in Europe. A systems error had added a sum of up to €500,000 on the bills of about 70,000 customers following a software upgrade, she said.
The company fixed its billing system yesterday by applying a software patch and is confident that no more errors will occur, she added.
It is understood the the Commission for Communication Regulation (ComReg) is deeply concerned that O2 did not inform the regulator about the problem despite uncovering it in April.
In a press statement, the ComReg chairman, Mr John Doherty, said he was concerned that O2's internal safeguards had not prevented the overcharging. He asked O2 to provide a report on the circumstances that led to the overcharging and provide details of what measures it had put in place to prevent any recurrence.
Ms Gray said the company always considered its own customers first and had wanted to fix the problem before telling customers or ComReg about it.
But she said that in the future ComReg would be on its checklist if a similar problem arose.