The network problems that hit thousands of O2 customers have continued into a second day.
However the company said its 2G network had now been restored and that customers should now be able to make and receive calls.
The widespread issues began “at lunchtime” on Wednesday which meant many lost voice and data services.
No timescale has been given for when the network will be fully back up and running.
In a message published at 08:00 GMT on its network status site, O2 said: “We can confirm that our 2G network service has now been restored. Customers who were affected should now be able to make and receive calls.
“Our 3G service is starting to restore and customers should expect to see a gradual return of data services as the day progresses.
“Customers affected may wish to try switching their mobile phones off and on as service returns.”
It apologised again for the inconvenience that was being caused. No details have been given about what has caused the network problems.
Customers of other firms that use O2’s mobile masts, such as GiffGaff and Tesco Mobile, were also still affected.
Tesco confirmed that many of its users were also having problems and cited issues with O2’s “core network”.
Spotty service
It is not clear how many of O2’s 23 million customers have been hit by the network fault but the operator said the problem was not based on geography. Across the country some will be able to connect at the same location as others who cannot.
“We can confirm that the problem with our mobile service is due to a fault with one of our network systems, which has meant some mobile phone numbers are not registering correctly on our network,” said a service update published about 12 hours after the problems had emerged.
“As a result, some customers are having difficulty making or receiving calls, sending texts or using data.
“We, and our central supplier, have deployed all possible resources and are working through the night to restore service as soon as possible. We apologise again to customers affected and will provide further updates as soon as we can”
Many users have sent messages via Twitter to complain about the issue.
“Starting to get silly now, six hours without service isn’t good enough in this day and age,” wrote Coronation Street actor Chris Fountain.
“Leaving you the first chance I get,” messaged another of the firm’s customers.
One user joked: “What did one O2 customer say to the other? Nothing!”
But others said they were unaffected.
“Everyone’s complaining about O2 reception, mine’s been working fine all day,” said one tweet.
Customers have been told they can keep track of developments via O2’s service status page.
Some have complained that the page is slow to load and sometimes fails to appear. A spokeswoman for O2 apologised and said the site was facing high demand.