Eircom has confirmed that up to 30,000 customers will be sent backdated demands for payment due to an error in its bill collections system.
The company said a systems error has resulted in thousands of direct debits not being collected for several months.
It said 30,000 people did not have some or all of their monthly direct debit payments taken from their bank accounts for phone, broadband and TV service since January.
It said affected customers received their bills as usual, which incorrectly stated that their bills had been paid by direct debit.
Eircom said those affected will receive a letter advising them of the error.
After that, the next bill will reflect the full amount due to be paid via direct debit.
The company said a dedicated phone line has been set up to answer queries. The number - 1800 303 432 - is available Monday to Friday, 9am-8pm.
Eircom said the error was related to the implementation of the Single Euro Payments Area (SEPA) payments.