O2 have confirmed on their blog and network status page that 2G network services have been restored, and all customers can now make and receive calls. It is recommended that you power your phone off and then on again to correctly log back onto the network.
2G data is also working, so GPRS and EDGE data services will work fine, although to ensure this functions you might want to disable 3G (where possible) to ensure your phone hooks onto 2G correctly.
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 some customers and can provide reassurance that we continue to deploy all possible resources, and will do so until full service is restored.