Thousands of O2 mobile customers have been without service for hours after a crash in the firm's network.
Customers have been enduring network loss since Wednesday lunchtime but the company has been unable to explain the cause.
The problem, which extends across the country, is not affecting all customers, the network said.
Some customers have been unable to make phone calls and some have had problems using data services.
A spokeswoman said: "We are currently seeing a problem on our network affecting some of our customers who will have difficulty making or receiving calls, sending texts or using data.
"We apologise to those customers who are inconvenienced by this.
"Our engineers are dealing with the problem as a priority and we hope to restore full service as soon as possible."
GiffGaff and Tesco Mobile customers, who rely on O2's masts, have also been affected.
An O2 spokeswoman said the problem was not restricted to one region: "The problem is an issue within part of our core network that is preventing some mobile phones from successfully connecting.
"The problem is not location-specific. All possible resources across our and our suppliers' engineering teams are being deployed to restore service as soon as possible."
Customers have taken to Twitter over the outage.
One Twitter user, Kelly Jones (@kelly_92), tweeted: "Having a phone that hardly works usually is annoying, but this whole no signal on o2 all afternoon is beyond irritating."
Another, Bethany Kemp (@Krazy_Kempy), said: "It's ridiculous, haven't had a signal since 1pm. @O2 you are going to have to do some apologising if you don't want to lose customers."
While some saw the humorous side, @Gazbelfast tweeting: "What did one O2 customer say to the other? Nothing."
O2 have been keeping customers updated through their service status page .
O2 have around 23 million customers in the UK.
It is unclear exactly how many have been affected by the problems but a spokeswoman said she estimated it was "thousands" at least.
The company was formed in 2001 following the demerger from British Telecom of its former mobile business, BT Wireless.
