O2 disruption: Why did the network fail?
12 July 2012 Last updated at 11:07 BST
The network problems that hit tens of thousands of O2 customers have continued into a second day.
The mobile firm said its 2G network had now been restored and that customers should now be able to make and receive calls.
Technology Correspondent Rory Cellan-Jones explained why the O2 mobile network suffered disruption.