It appears that O2 have another signal/network problem with numbers not working which is affecting Tesco Mobile users - apparently affecting phone numbers across the UK as it isn't limited to a specific region. Despite their network problems
O2 is still one of the cheapest places for the new Nokia Lumia phones
Update on O2 Network Problems: 12 Oct 2012 @2100
Apparently a lot of customers are able to get an O2 signal back by turning off the 3G connection on their phones. If you then use WiFi you can still access data services and make calls so it appears there is a problem in the O2 3G network again.
If you're still bored of waiting for your O2 signal to come back you can always
check out the modelling photo gallery!
Update: 12 October 2012@ 1908
Problems are continuing - customers all over the UK are affected but not all O2/Tesco customers. 2 Phones in the same location may have one working and one not so it appears to be some form of corruption or error in the O2 database that manages network access which was the same problem that happened in July 2012.
Update: 12 October 2012 @ 1700
We are aware of Tesco mobile customers across the UK that have problems with no signal and unable to make calls from Kent to Birmingham to Scotland.
The details are on the status page of the O2 website but hopefully will not be as major as the outage in July 2012 that affected millions of Tesco Mobile users over 2 days.
You can read about the O2 Network downtime in July 2012 here