O2 (UK) they can do it too" in reference to Vodafone and EE, O2's biggest competitors and the UK's top three mobile-telecoms companies. 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
O2 (UK) computer monitor standby times. O2 is a voluntary participant in the climate change campaign, which required participants to cut their carbon emissions by 10 per cent by the end of 2010, and has since broadened its approach to include a range of projects focused on carbon reduction and renewable energy. In October 2017, an inquiry was launched in the UK concerning overcharging customers for handset sales integrated with the data and call charges. Responding to questions, O2's CEO indicated that the company's billing allows for separation of handset charges and telephony/data charges. He added that "if we can do it,