Engineers advise turning off 3G as a stopgap fix.
The network issues plaguing UK O2 customers have stretched into a second day.
While the 2G network has now been restored, there are still problems with 3G. The company is advising users to turn of their 3G settings, which should give them some coverage.
Customers of other firms that use O2's mobile masts, such as GiffGaff and Tesco Mobile, were also still affected.
It's not yet certain how many of the firm's 23 million customers have been affected, but social networks like Facebook and Twitter are alive with complaints and demands for compensation.
This morning, 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."