The majority of Smart Telecom's customers are no longer able to make outgoing calls from their phones tonight because of a dispute with the company and its wholesale provider Eircom.
Smart customers are being advised to contact ComReg. They can still receive incoming calls and are able to make emergency 999 calls.
An Eircom spokesman said that the company has had credit issues with Smart Telecom for some time.
A small number of Smart customers - those who still pay their line rental to Eircom - can still make outside calls by using the pre-fix 13666 before the telephone number they want.
A spokesman for Smart Telecom says the company regrets the inconvenience to its customers and is currently attempting to rectify the situation.
In a statement tonight, Eircom said it regrets that it has been forced to cease providing services to Smart Telecom in response to serious credit issues.
It added that it will work closely with the regulator and the industry to assist in whatever way possible to help affected customers.
A spokesman for ComReg, the telecommunications regulator, said tonight it is regrettable that Smart customers have had their services affected because of today's events.
ComReg said it will put information on its website for affected customers and will take out ads in the national newspapers to inform affected Smart customers of what steps they should now take. The ComReg website is www.askcomreg.ie
Smart Telecom has been in financial trouble lately. Last month, it said it had concluded a strategic review which would see about 250 jobs being cut as the company's staff numbers are reduced from 348 to about 100.
It also said it planned to sell off a number of non-core businesses - including payphones and pre-paid call cards - in the coming months.
