Thousands of mobile users across the country have been unable to send or receive calls or texts after O2's network crashed.
The problem, which extends across the country, is not affecting all customers, the network said.
Some may also have 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."
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."
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.
A spokeswoman said the problem was not location-specific: "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."