O2's 3G data network remains down for hundreds of thousands of people, almost a day after those customers lost all voice, text and data connectivity.
The restoration of 2G voice and text was announced at 8am on Thursday morning, with the initial reports of downtime having come late on Wednesday morning. The operator has advised affected customers to turn 3G off on their phones, so that they can get consistent services through 2G.
"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," a status update read. "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 some customers and can provide reassurance that we continue to deploy all possible resources, and will do so until full service is restored."
ZDNet understands that hundreds of thousands of customers are affected, including not only some of O2's 23 million customers, but also those of Tesco Mobile and GiffGaff, which are 'mobile virtual network operators' (MVNOs) that effectively piggyback on O2's network.
An O2 spokesman told ZDNet on Thursday morning that it was not possible to divine the numbers of affected customers at the time, given that the problem was not related to a specific geographical area or number range.
All that O2 has been able to explain by this point is that the problem "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 are working through the night to restore service as soon as possible," a status update from the operator read at 1am. The O2 network status page has apparently been crashing intermittently under the strain of people trying to see what is wrong with the network.
The operator has not yet addressed the issue of compensation for the outage.
Update (10am): Analyst firm Ovum has offered its take on the situation, which (although the cause of the outage remains unclear) is that it might indicate problems in two weeks' time, during the London 2012 Olympics.
"The huge influx of visitors to London ahead of the games, will cause network traffic spikes, putting pressure on the UK's mobile networks, which already have a poor reputation compared to others in Western Europe," analyst Stephen Hartley said in a statement. "While UK mobile operators claim to be prepared, they have not yet given indication of the scale of their plans."
"We hope that our concerns are unfounded and the Games pass without telecoms incident. Connectivity at the Olympic Park can at least be predicted and planned based on a greenfield implementation of the latest technologies and architectures. However, the UK as a whole must rely on previous investments to carry it through — and these have been less than optimal in the mobile space."
UK authorities had previously suggested that the Olympic load would cause broadband outages, but did a u-turn last week, saying everything will be fine.