O2 Network Down with No Signal (so also affecting Tesco Mobile and GiffGaff)
If you are having problems please leave a comment to help show what areas are affected across the UK and how many people are affected by this problem with O2 which also covers Tesco Mobile and GiffGaff phones which use the O2 network.
UPDATE - 0845 on Thursday 12th July 2012
O2 have stated that the problem is "affecting thousands". Based on the number of people I know on O2 that are still unable to make calls this is a gross underestimate of the numbers affected. I would guess it is in the millions not thousands.
The problems are still continuing today and from Manchester to London to Eastbourne & Dover O2 mobiles are still unable to make calls. Some are showing full signal but still not able to make calls or send texts. Others have no signal at all. O2 have stated the problem is due to a failure in their core network but it has also now caused their website to crash so users are unable to check status updates.
One suggestion to resolve the problem has been to turn off data roaming if you have a smartphone like the iPhone but this may not work with all affected phones. Someone else has suggested that one factor may be whether phones have an original O2 number or were migrated from other networks but this may not be a common factor.
O2 have issued a statement that the problems are due to a failure in their core network. Friend's mobiles are still showing no signal nearly 12 hours after it started.
O2 Network Down (Again) with No Signal