Thousands of O2 customers across the country were still unable to use their mobile phones today as the company continues to try and fix the fault.
Engineers worked through the night but many mobile users are facing a second day of being unable to make or receive calls.
Text messages are also affected by the fault, while customers have also experienced issues accessing data services such as the internet.
A company spokeswoman said:
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 how many had been affected by the nationwide problem, but estimates it was at least "thousands". O2 have around 23 million customers in the UK.
A message on O2's Twitter feed read said the 2G network has now been restored and they are working on the 3G service.
Alice May Jones: My signal is back but it just says 'call failed' when I try to ring my boyfriend who is in the other room! It's obviously not fixed!
Marie Cheetham: Mine is not working. It's not doing anything, texting or ringing. I turned it off and back on like they said and still nothing.
Celebrities have gone on Twitter to mock the mobile network:
O2, which was formed in 2001 is urging customers to check their Twitter and Facebook feeds for updates.
Tesco, which uses O2's masts for its Tesco Mobile network, confirmed that some of its customers were being affected this morning.
We are currently seeing a problem on our network affecting some of our customers. There is an issue within part of O2's core network that is preventing some mobile phones from successfully connecting. We're sorry about the inconvenience this may cause our customers, and O2 engineers are dealing with the problem to restore full service as soon as possible.