O2 (UK) O2 (UK) Telefónica UK Limited (trading as O2 – stylised as O) is a telecommunications services provider in the United Kingdom, owned by the Spanish multinational Telefónica, and is headquartered in Slough. With 25 million subscribers (as of December 2017), O2 is the second-largest mobile network operator in the United Kingdom after EE, with Vodafone in third place. O2 was formed in 1985 as Cellnet, a 60:40 joint venture between BT Group and Securicor. In 1999, BT Group acquired Securicor's share of Cellnet and the company was later rebranded as BT Cellnet. In June 2000, BT Cellnet launched the world's
O2 (Ireland) available to customers paying similar amounts in the UK. Less than 48 hours after the much hyped launch of Apple's latest model, owners of all phones on the O2 network discovered they could not access the internet at all due to a network failure. Further criticism came from iPhone customers regarding the continued failure of O2 to deploy Visual Voicemail. O2 was the only carrier in the world to launch the original iPhone without Visual Voicemail, one of its headline features. O2 was responsible for the highest number of registered judgments, which financially blacklists those people who do not pay