Part of the Telefónica group, O2 is one of the UK's largest mobile networks, renowned for its impressive perks and extras.
Choose O2 and you get a massive choice of mobile phones - from budget options to top end devices like the latest iPhone - alongside a variety of pay monthly and pay as you go plans that cater to all budgets.
It also gives customers lots of benefits, including upgrade programs, priority tickets for O2 events, and more.
In 1985 BT and Securicor launched Cellnet as a joint venture. 14 years later, Securicor sold its remaining shares to BT, the operation was then renamed BT Cellnet - though in 2001, the company separated from BT and became O2 UK. A year later it launched the first 3G network on the Isle of Man.
In 2005, O2 was acquired by Spanish company Telefónica for a reported £17.7 billion. O2 retained its name, management team and its UK location.
In addition to O2, Telefónica Europe also owns 50% of Tesco Mobile - which also uses the O2 network.
O2 also offers a nice big range of monthly and Pay & Go SIM-only deals, starting from £8 for 12 months. As an O2 customer you can also benefit from priority tickets to events and other services such as free back up, and O2 Rewards.
It also offers O2 Refresh contracts, which make it easy to upgrade your phone. These split your monthly bill into the cost of your handset and the cost of your tariff - meaning you can pay off your handset and upgrade it, or switch around your monthly allowance, whenever you want.
O2's overall network covers 99% of the UK population; its 3G covers over 85% of us; and 4G coverage is a little lower.
Telefónica UK Limited
PO Box 202
For pay monthly: 202 from an O2 mobile, 0344 809 0202 from another phone, +44 344 809 0202 from abroad
For Pay & Go: 4445 from an O2 mobile, 0344 809 0222 from another phone, +44 786 098 0202 from abroad
Lines are open Monday-Friday 8am-9pm; Saturday 8am-8pm; Sunday 9am-6pm