Thousands of O2 Ireland customers are being refunded for roaming fees they were charged while travelling in Northern Ireland over the past six months.

However, the firm denied yesterday that it had overcharged customers, and does not plan to refund people who were charged the roaming fees before April.

O2 Ireland is refunding subscribers who signed up for its All Ireland tariff, which is meant to enable its customers to make calls without incurring roaming fees while in Northern Ireland.

However, the special tariff, which was introduced in September 2003 and costs €5 per month, does not work for any calls to numbers in Northern Ireland when the international prefix 0044 is used.

Standard call rates were only charged for subscribers travelling in Northern Ireland who used the local access codes.

A spokeswoman for O2 Ireland said the terms and conditions clearly stated this fact to its subscribers, but admitted many customers had been confused by it.

She said the firm had decided to extend its All-Ireland tariff to include calls made to numbers using a 0044 prefix from October after realising the confusion.

It was also crediting the difference between the roaming rates and standard call rates to O2 Ireland customers who made calls using 0044 over the past six months.

Roaming fees are charged at substantially higher rates than standard mobile phone charges, and customers could have been charged up to 32 cents more per minute of use for their calls.

O2 Ireland did not mention the refunding of All-Ireland customers when it attended an Oireachtas inquiry investigating overcharging in November.

The O2 spokeswoman said the firm did not communicate with its customers via either the Oireachtas or journalists. It contacted them itself, she added.

The two biggest mobile phone operators, O2 and Vodafone, have been plagued by billing errors this year, resulting in overcharges of hundreds of thousands of their customers.

In July, The Irish Times revealed that almost 140,000 O2 customers had been overcharged €721,892 by the firm for calls received while travelling abroad.

The firm said the overcharging incident related to a system error which resulted in its customers paying twice for calls received while they travelled overseas.

The error occurred following a software upgrade at the company and also a change in its roaming tariffs.