The linear correlation between violent crime rate and percentage of the population that has a cell phone is minus0.918 for years since 1995. do you believe that increasing the percentage of the population that has a cell phone will decrease the violent crime rate? what might be a lurking variable between percentage of the population with a cell phone and violent crime rate?

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.