Telefónica UK Limited (trading as O2) is a telecommunications services provider in the United Kingdom, owned by the Spanish multinational Telefónica, and is headquartered in Slough. With 26 million subscribers (as of November 2019), O2 is the second-largest mobile network operator in the United Kingdom after EE, with Vodafone in third place. The company was formed in 1985 as Cellnet, at first a joint venture between BT Group and Securicor, later wholly owned by BT. Cellnet was the second mobile phone network to launch in the UK after Vodafone three years earlier. It was spun off from BT in 2002 to form a new holding company, mmO2 plc, which introduced the "O2" brand for the businesses. In 2006 the company was purchased by Telefónica, who retained the O2 brand and the company's UK headquarters.