4:00 AM -- Customers of Telefónica UK Ltd. 's O2 have been unable to make calls, sends texts or use data services since Wednesday afternoon following a widespread network outage. The problem started around 1:00 p.m. on Wednesday and appeared to have been resolved (at least for some) shortly after 9:00 a.m. Thursday (local time).

O2, which has about 23 million customers, provided the following explanation for the service outage early Thursday morning: "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. As a result, some customers are having difficulty making or receiving calls, sending texts or using data."

Not only that, customers were also having difficulty even getting to that online explanation, as that Web page soon became unavailable (at least temporarily), most likely due to the volume of access requests. The operator also posted updates on this blog.

An O2 spokesman declined to provide further details when asked about the service outage by Light Reading Mobile.

It's not known precisely how many customers have been affected, though it's clear that O2 customers throughout the U.K. were affected. Customers of O2's MVNO partners, such as GiffGaff or Tesco Mobile, have also been affected, according to this BBC report.

O2's disruption follows the major network outage that Orange France suffered last week, where 30 million customers were left without service for 12 hours. According to Orange, a software fault in Alcatel-Lucent (NYSE: ALU) equipment caused problems on a home location register (HLR).

O2's next challenge will be to placate its customers, many of which will be considering whether the operator can be relied upon to maintain a constant service in the future. Orange France has stated that it will be providing a day of free calls and texts and a free data usage top-up in September as compensation for its outage -- what will O2 do?

— Michelle Donegan, European Editor, Light Reading