O2 (www.02.co.uk) launched in 2002 and with over 21 million customers in the UK, now has more than any other network.
You can compare O2 mobile deals and then sign up to any O2 Pre-Pay, Pay & Go or Sim Only tariff online or in any one of the companyís high street stores.
As well as mobile phone deals, O2 also offers a range of other products, including mobile broadband and home broadband bundles.
O2ís mobile phone network covers 99% of the population, and its 3G network - which is fully HSDPA (High-Speed Downlink Packet Access) enabled - covers more than 84% of the UK.
Its 3G network allows you to get online with mobile broadband and make video calls from your phone with speeds of up to 3.6Mb.
O2 launched its home broadband service in 2007, going on to win a host of awards. It offers speeds of up to 8Mb or 20Mb with great discounts for existing O2 mobile customers.
It also offers mobile broadband deals, and in March 2010 it launched its latest service, with a choice of two different home phone plans, starting at just £9.50 a month including line rental.
As well as being able to bundle your O2 mobile phone with other services, you can also take advantage of a great range of added extras as an O2 customer.
From the O2 Blueroom, which offers priority tickets sales, to its Bluebook backup service that stores your texts, photos and contacts for free, O2 has a huge range of useful, money-saving services to choose from.
A free iPhone app is also available that lets O2 customers search for priority tickets for music gigs, shows and events directly from the Apple smartphone.
It also regularly offers deals on bolt-ons such as extra text allowances, data bundles .