The network problems that hit hundreds of thousands of O2 customers have continued into a second day.
The mobile firm said its 2G network had now been restored and that customers should now be able to make and receive calls.
O2 is advising users to turn off their 3G setting while engineers work to restore that part of the network.
The widespread issues began "at lunchtime" on Wednesday which meant many lost voice and data services.
In a statement the firm 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."
It was advising customers still affected by the outage to switch their mobile phones off and on.
Few details have been given about what has caused the issues.
"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.
It is not clear how many of O2's 23 million customers have been hit by the fault but the operator said the problem was not based on geography. Across the country some will be able to connect at the same location as others who cannot.
Customers of other organisations that use O2's mobile masts, such as GiffGaff and Tesco Mobile, were also still affected.
Transport for London confirmed that the network issues had affected some docking stations for "Boris Bikes", the scheme which allows people to hire cycles around London.
"We are aware of this problem, and we're currently working to resolve it as soon as possible," TfL said in a statement.
O2 users contacted the BBC to describe how the glitch had affected them.
Joanne Clarkson from Newcastle said it had severely disrupted her day: "Being the mother of a child with special needs I need my phone. Until this is fixed I will have to stay home so I can be contacted by my landline if the school needs me," she said.
Chris Benson, owner of Crystal IT services based in Barry, Wales said that it had affected his business: "It's a little difficult to direct our workforce, contractors or suppliers when they all have to rely on landlines."
Others have expressed their anger via Twitter.
Customers have been told they can keep track of developments via O2's service status page .