Thousands of people affected by the O2 blackout have found they are able to make calls and text messages if they turn off 3G mobile data.
O2 is still unable to say when its network will be fully restored after disruption hit hundreds of thousands of its 23 million UK customers on Wednesday afternoon and overnight.
But the company said on Thursday morning that users should be able to make calls and text messages, and use 2G data, if they switch off the 3G option for mobile data use.
The mobile network said in a statement: "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. We are sorry again for the inconvenience this has caused and can provide reassurance that we continue to deploy all possible resources and will do so until full service is restored."
O2 customers found they were unable to make calls or send and receive text messages from about mid-afternoon on Wednesday. The disruption continued throughout the night and into early Thursday morning.
However, several customers told the Guardian they were still unable to send or receive calls about 9am on Thursday.
Some O2 customers who were not affected yesterday also claimed to have been disrupted on Thursday, in a sign that the network outage spread further overnight.
One customer, Jo Phipps, posted on Facebook: "My o2 phone been down for 6 hours ... whats going on?"
Another user, Gaz, had a more humorous view of the disruption, tweeting: "What did one O2 customer say to the other? Nothing."
BBC television presenter Huw Edwards tweeted: "6 hours of non-service and counting, simply not good enough, O2."
O2 is urging customers to check their Twitter and Facebook feeds for updates, but the company's webpage – which displays live information about network coverage – has crashed.
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