Some 3,000 Eircom customers will shortly be issued refunds after they were overcharged by €25-€300 for a call barring service that prevents calls to premium rate phonelines, mobiles and personal numbers.

The mistake was discovered as part of an audit of Eircom's billing systems that had been ordered by the Commission for Communications Regulation (ComReg), after it emerged in 2004 that the fixed-line operator had overcharged 31,500 customers for call management services.

"An overbilling issue arose three years ago and ComReg requested Eircom to conduct an audit of its systems," a spokesman for the regulator said yesterday.

"These affected customers are being reimbursed as a result of that audit and ComReg has been in contact with Eircom to ensure that everyone who was affected has been reimbursed."

Eircom apologised for the mistake and said it was in the process of issuing letters to the customers who were overcharged.

A spokeswoman for the company said the charging error had occurred as a result of a system glitch that had resulted in the duplication of some charges.

Most of the customers who were overcharged will receive refunds closer to the €25 mark, she added.

Eircom said that the technical problem had now been fully rectified.

In 2004 the company admitted that it had been overcharging 31,500 customers an estimated €409,000 for services, including call barring, call answering and call diverting since 1992.

That incident prompted ComReg to call for a full report outlining the mistakes that had arisen, why internal safeguards had failed and what steps the company had since taken to prevent similar errors.

Call barring is a subscription-based service offered by Eircom, with eight different levels, depending on the types of calls that the billpayer wishes to block.

The service has a once-off set-up charge of €20.32, excluding VAT, except in the case of consumers who only want to bar calls to premium-rate numbers. The service then attracts a monthly charge of €2.54-€3.81, excluding VAT.

Call barring is offered to both business and residential customers. It is sometimes used by employers who wish to stop employees making calls to premium rate phonelines or in cases where there is shared use of a residential landline, for example where a number of tenants are renting a property but there is only one billpayer.