Cellnet, jointly owned by BT and Securicor, proved the UK's fastest growing mobile network and is investing a further £700 million to create one of the world's most advanced mobile phone networks. Today there are more than 5 million mobile users in the UK - with almost 2.4 million Cellnet customers. Cellnet's new digital network conforms to the Global System for Mobile Communications (GSM) standard, which enables customers to use their phones in more than 35 countries across more than 50 networks.