O2 mobile users regained a partial service on Thursday after a fault left customers of Britain's second biggest mobile network unable to use their phones for almost a day.
"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," the company said on its website.
"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 phonesoff and on as service returns."
Engineers had worked through the night trying to resolve the network problem, which affected an unknown number of O2's 23 million British customers as well as users of Tesco Mobile.
Media reports said hundreds of thousands of phone users were hit by the fault, which began on Wednesday afternoon.
The fault had disabled text messaging and data services as well as calls, with a total outage for some users and an intermittent service for others.
Users needed to switch off their 3G connection in order to access the newly-working 2G link, according to customers who took to Twitter and Facebook in search of answers.
TV presenter and choirmaster Gareth Malone tweeted: "Day two without phone reception. The quiet is disconcerting now. What if there's an emergency? I haven't used a phone box since 1998."
A spokeswoman earlier said the problem was not location-specific and had affected "thousands" of users.
O2, which was formed in 2001, following the demerger from British Telecom of its former mobile business BT Wireless, and is now owned by Spanish firm Telefonica.