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
Rochester Telephone Corporation for resale. In their early years, the company's subsidiaries reported strong profits. In 1983, as the federal government's breakup of the Bell telephone monopoly increased competition in the telecommunications industry dramatically, Rochester Telephone launched its second unregulated venture, founding the RCI Corporation. RCI installed a microwave communications network in New York State and a fiber optic hookup with Chicago. After purchasing an additional fiber optic link with Washington, D.C., the company's operations spanned 12 cities. In the mid-1980s, Rochester Telephone also entered the new radio telephone industry, when it founded Rochester Tel Mobile Communications. By the end of the decade,