O2 says it has fixed network problems that left hundreds of thousands of customers without voice and data connections.
“Our tests now show that all our 2G and 3G services have been fully restored for affected customers,” the firm said in a statement.
“If any customers are still having problems we recommend they turn their phone off and on again.”
The news came about 25 hours after O2 had said the problems first emerged.
Subscribers to GiffGaff and Tesco Mobile – services which use O2′s mobile masts – had also been affected.
“Once again, we are sorry,” added a spokesman.
Little has been revealed about what caused the problems, although the mobile firm has narrowed it down to the part of its network which deals with registering telephone numbers.
“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 the firm.
Steven Hartley, principal analyst at Ovum thinks the most likely cause is a software failure.
“It seems likely that it may related to the Home Location Register, which is used to check that someone is a legitimate customer. It must have been two separate boxes if both the 2G and 3G network fell over,” he said.
Askar Sheibani, chief executive of IT and telecoms repair company, Comtek said the crash shows the importance of basic network maintenance.
“To have an entire network down is a poor show for the company, and does not bode well for an operator who plans to deliver 4G services in a year’s time,” he said.
“This failure to deliver basic 2G and 3G services, indicates that O2 may have taken its eye off the ball – ignoring the maintenance of its existing service in the rush to deliver 4G,” he added.
O2 is not the only network operator to be experiencing problems.
France Telecom’s mobile network Orange experienced similar issues last week.
In testimony before France’s National Assembly on Wednesday, France Telecom chief executive Stephane Richard said that the crash was probably caused by a software upgrade of Alcatel-Lucent equipment 48 hours earlier.
The crash was the biggest disruption of communications seen in France since 2004.