Wikipedia reports:

Vodafone have reported that their mobile phone network reached capacity at about 10 AM on the day of the incident, and they were forced to initiate emergency procedures to prioritise emergency calls. Other mobile phone networks have also reported failures. London's Transport system is currently paralysed, significantly because of the complete closure of the underground system and the Zone 1 closure of the bus networks, as well as evacuation of Russell Square. The BBC has speculated that the phone system has been closed by the security services to prevent the possibility of mobile phones being used to trigger bombs. BBC reports mobile networks are running again.

• 'Norwegian Voices' - or how to spoil a great opportunity - Dec 13, 2005
• One guy caught? - Jul 21, 2005
• New London blasts - where to get more information - Jul 21, 2005
• Hackney Road bus blast - Jul 21, 2005
• Multicultural London - Jul 07, 2005
• See more related posts in: London