Hundreds of thousands of customers facing a second day of disruption have found they are able to make calls and send texts if they turn off 3G mobile data.
An O2 spokesman confirmed that it was encouraging those with smartphones to turn off their 3G capability to regain basic service, saying that its 2G network had been restored.
The spokesman said 3G was "starting to restore" but he could not give a timeframe for when customers could expect to see the entire service back to normal.
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.
"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."
Engineers worked through the night to fix the fault after network problems started to be reported early yesterday afternoon.
A company spokeswoman said earlier: "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
"We, and our central supplier, have deployed all possible resources and have been working through the night to restore service as soon as possible."
The spokeswoman said it was unclear exactly how many have been affected by the nationwide problem, but estimated it was "thousands" at least.
Michael Allen, Director of IT Service Management at Compuware said, “In recent weeks we’ve seen how technology problems can have significant problems on the everyday lives of millions of people. First we saw the problems at NatWest and RBS and now 02. Not being able to make a call will be as serious to many people as not being able to take cash out of the bank. Unfortunately, these problems will only continue to increase unless organisations take a fundamentally different approach to the way they manage the performance of the IT systems we rely on to go about our day to day lives. Too often institutions are managing the technology stack in silos. The problem is, the systems that make up these day to day services are getting more and more complex and interdependent.
O2’s ability to deliver a service to customers will rely on hundreds of different components, systems and applications working in harmony. This can make preventing these types of service disruptions difficult as well as finding the root cause time consuming. This is why a new approach needs to be taken; companies must manage their technology services in a much more integrated and holistic service centric manner.”