Eircom says it is working on a major issue which has left hundreds of thousands of people without phone lines.
Mobile and landline services for some businesses which use 1800 and 1500 numbers - as well as ISDN lines - have been down since just after 10am today.
Eircom says standard fixed line numbers are unaffected and that the problem is intermittent.
Customers experiencing problems cannot call the customer care centres - so are advised to report the fault online.