Several countries, including the UK, now have more mobile phones than people. There are over five hundred million active mobile phone accounts in China, as of 2007. Luxembourg of the old school the highest mobile phone penetration http://www.phonesmobile.org estimate in the world, at 164% in December 2001.
An lesson of criminal investigations using mobile phones is the initial location and ultimate identification of the terrorists of the 2004 Madrid train bombings. In the attacks, mobile phones had been used to detonate the bombs. However, solitary of the bombs failed to detonate, and the SIM card in the corresponding mobile phone gave the first serious pilot about the terrorists to investigators. By tracking the whereabouts of the SIM card and correlating other mobile phones that had been registered in those areas, police were able to locate the terrorists.