Thousands of O2 mobile customers in the U.K. are without service this evening following an unidentified network outage.

Customers were informed on Twitter just after 4:30 p.m. BST that a "problem" is affecting some customers' mobile service --- including making calls and using data --- and that engineers were on the case.

There’s a problem affecting some customers’ mobile service. Engineers are working to restore full service asap. j.mp/O2Coverage — O2 in the UK (@O2) July 11, 2012

But the network status page said the outage started at 1:30 p.m. BST, three hours after O2 took to Twitter to inform users of the outage.

Many also took to the microblogging site to complain that not enough details were issued, despite O2's stream of apologies in reply to irate customers. Some also tweeted the fact that their network service was unaffected.

An O2 spokesperson was unable to say how many were affected by the outage, but that it was "not regional." This is backed up by various related tweets from as far as North Wales and the Scottish Highlands, as well as tweets in the south-east of England.

"We are currently seeing a problem on our network affecting some of our customers. Those customers affected will have difficulty making or receiving calls, sending texts or using data," the spokesperson said.

'Virtual' networks GiffGaff and Tesco Mobile, who piggyback off O2's infrastructure, were also hit by the outage.

This is the second outage in as many weeks. Towards the end of June, thousands of O2 customers were unable to send text messages for nearly a whole day, reports The Telegraph.

Telefonica-owned O2 has more than 23 million mobile, landline, and broadband customers in the U.K.

For now, if you want to contact me, I guess there's always email, but feel free to send me a letter, a carrier pigeon, or smoke signals if you're in or around the Canterbury area.

Update at 12 July 7:50 a.m. BST: Many O2 customers --- including this one --- are without service. To add insult to injury, the O2 Status page is showing intermittent load problems due to the high demand of traffic. O2 took to Twitter to inform its customers, but the last update was around 9:20 p.m. BST last night.

Update at 8:00 a.m. BST: A statement from O2 says 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."

Some are finding data services to be back up and running, while dialling out is still impossible for many.

Update at 11:00 a.m. BST: O2 said it expects to restore "full service" to all affected customers this afternoon. It also said its 2G network "has been restored," and advises customers to switch of 3G access to access the mobile Web. CNET's Charlie Osborne has more.

Update at 2:30 p.m. BST: An O2 spokesperson said "Our tests now show that all our 2G and 3G services have been fully restored for affected customers," the firm said in a statement," and added: "We are sorry."