Mobile giant Vodafone has admitted to overcharging 100,000 of its Irish customers a total of €900,000 by incorrectly billing them.
The customers were billed for having their incoming calls diverted to voicemail when they were outside the country.
The company, which is Ireland's biggest mobile operator, has begun refunding affected customers, while those who are due refunds of more than €30 are being contacted directly by the firm to explain the error.
In 2007, Vodafone announced that both prepay and bill-pay customers would not be charged while roaming if they received a call that was diverted to their voicemail.
This was an effort to help customers reduce their phone charges while abroad.
Vodafone said that the implementation of the new system required extensive testing over the course of a year before it was introduced in October 2007.
However, last November a Vodafone customer queried a charge on a bill, prompting the company to initiate a detailed trawl through its network to identify the problem.
It emerged that 100,000 bill-pay customers had been overcharged as a result of a technical glitch.
Vodafone declined to say what the largest amount owed to a single customer is following the overcharging, but added that for 60pc of the affected subscribers the amount they're owed is less than €6.
The company apologised for the mistake yesterday, saying that it was sorry for any inconvenience caused to its customers.
Telecoms watchdog Comreg said yesterday that it had been made aware by Vodafone of the overcharging and that a comprehensive report had been provided which would be analysed.
In 2004, Vodafone admitted it had overcharged more than 20,000 customers for their roaming costs over a one-month period, while rival operator O2 also confessed at the time that it had overcharged 70,000 customers for their calls while they were roaming. Vodafone said at the time that it had introduced new roaming charges that had been miscoded in its billing system, leading to the excess charges. About €147,000 was refunded to customers.
Later that year, Vodafone revealed that about 550,000 of its customers using the WAP internet function on their phones had been overcharged a total of €2.65m. That overcharging was uncovered after Vodafone called in accountancy firm PriceWaterhouseCoopers (PwC) to examine its billing systems. About half the affected customers had been overcharged by less than €1.
Vodafone, which has over 2.2 million subscribers here, made a profit of €270m in Ireland in the 12 months to the end of March 2008. It owns the Perlico telephone business, which it bought in 2007 for €57m.
Vodafone Ireland's customers sent over one billion text messages in the three months to the end of December, up almost 21pc year-on-year.