Network Information

Networks and Operating Frequencies:
Mobile Phone companies work on various frequencies and various technologies throughout the world. The largest and most popular network is the GSM network based on the European developed standard. This works on 900mhz and 1800mhz frequencies.
In the UK Vodafone and O2 use 900mhz. Orange and T-Mobile use 1800mhz.

3G is the third generation technology in mobile phone communications allowing several MBps of transmission for the new multimedia applications on video phones.

UK Networks:

Vodafone
Vodafone began in 1985 (then Racal Vodafone) when the the mobile revolution began in the UK. They along with Cellnet were the two major networks that started the mobile phone revolution in the UK that year. They are the largest mobile company in the UK. Vodafone provide National UK coverage and work on the 900 MHz Network.

O2
The O2 UK network originally started out as Cellnet (owned by British Telecom) in 1985, later becoming BT Cellnet in 1999 but was demerged into mmO2 in 2001 and the network was called O2. In 2005 the company mmO2 plc reformed as O2 plc. This was the result of BT having to streamline their operations in a bid to control their costs. BT were now out of the mobile phone business but returned in 2003 with BT Mobile.
O2 provide national UK coverage and work on the 900mhz network.

T-Mobile
T-Mobile began as Mercury One2One in 1993. They were the third mobile phone network in the UK after Cellnet and Vodafone and the first to use the digital 1800mhz network while the competition were still using an analogue 900mhz service back then. They started out only within the London M25 area and then steadily grew outside and around other major towns and cities. One2One's major incentives were cheaper calls and special contracts offering free calls in the evenings. T-Mobile now provide national coverage claiming to cover 99% of the UK population. T-Mobile work on the 1800mhz network (PCN).

Orange
Orange began their 1800 mhz digital mobile phone service in 1994 soon after Mercury One2One. Orange, unlike One2One started as a national coverage network quickly building and improving their network over the next couple of years. Orange further brought the costs of mobile calls down with cheaper calls and per second billing. Before Orange introduced per second billing, all mobile calls were charged per minute (I remember my extortionate bills!). Orange were also the first to introduce Caller ID on their digital network, so you knew who was calling!
Originally owned by Hutchison Telecom (of Hutchison Whampoa of Hong Kong), Orange were sold to Mannesmann Tally for £20billion in 1999 and then acquired by Vodafone who bought Mannesmann Tally out. Orange is now owned by France Telecom who paid £25.1 billion to Vodafone in 2000.
Orange provide a national coverage and work on the 1800mhz network (PCN). 

Three
Hutchison 3G are the latest and fifth company to start their own mobile phone network in the UK in 2002. Marketed as 'Three' and owned by the same company that started Orange, Three have come into the market with some very generous phone packages and major advertising (just as they did with Orange) to get a serious share of the market. Three were the first to start with the 3rd generation technology known as 3G. 3G technology is the latest in mobile communications providing a far greater transmission rate allowing for rate hungry multimedia applications like live video and internet access.

Virgin
Virgin Mobile began in 1999 as a joint venture between Virgin and T-Mobile. Virgin was used for it's brand image providing it's own value for money mobile phone services having signed a 10 year contract with T-Mobile to use their network. In 2004 Virgin Mobile bought back all shares interests that T-Mobile once held. Virgin Mobile works on the T-Mobile network and therefore uses the 1800mhz network.

BT Mobile
In 2003 BT went back into the mobile phone market with BT Mobile. BT Mobile doesn't own it's own network, like Virgin, but sell it's mobile phone services using the networks of other mobile phone companies or also known as an MVNO - Mobile Virtual Network Operator.