This is a case of statistical correlation that does not correspond to causation. Those 2 variables are unrelated, hence an increase in the percentage of population that has a cell phone should not affect the violent crime rate. Nevertheless, there are other factors that affect both variables. For example, the development of technology has increased both the availability of phones and the difficulty of getting away with a crime due to modern forensic methods. Hence, this is a good example of a lurking variable.
