Eircom has admitted overcharging over 30,000 customers for its call management services.
The company said it discovered a fault in its billing processes following a customer query.
Eircom said the amount it incorrectly charged is estimated at €409,000 in total and to have affected 31,562 customers.
In a statement today, the company said it "regrets these errors and wishes to apologise to all affected customers".
It said it would refund customers in full as soon as possible and, as a gesture of goodwill, it would also apply the same amount again in credit to each customer.
The company said the issue came to light on July 23rd and the telecommunications regulator ComReg has been informed.
It said: "A thorough check of our records indicates that 18,291 customers who ordered call answering since June 1st
free of charge as part of our Talktime packages were incorrectly billed at an average charge of €1.57."
"Having identified this process fault, we have reviewed our charges in relation to call management services prior to June 2004," it said.
"This review has identified 13,271 customers who have been charged for duplicate call management services (this equates to approximately two per cent of customers who use call management services)."
Eircom said: "The average amount per customer is €28.62, the large majority of which arose post 1999.
Charges for call management services have been itemised on all customer bills since the introduction of the new eircom bill in 1999."
The company has set up a freephone number on 1800 201027 for customers with queries relating to the overcharging.
