When you open a new cell phone line in the United States, you (typically) get a cell phone number that has an area code of the area in which you currently live. Way back when before national roaming become the norm, people would often change phone numbers when they moved due to spotty carrier coverage; some carriers were better in some cities/areas than others and the place you move to may not necessarily have good cell phone coverage from your old service provider, hence you changed. This regular change of number meant your cell phone number’s area code typically provided accurate information about where you live. Nowadays, however, this no longer holds true. In fact, the following comic may put a smile on your face but it is fairly accurate:

[via xkcd]