In North America (mainly the United States and Canada), certain regions served by area codes experienced rapid expansion due in part to computer dial-up modems and the adoption of mobile phones. Instead of splitting the regions into different area codes, regions were assigned a second, overlaying area code. Examples of overlayed area codes are:
Miami, FL: 305 and 786
Los Angeles, CA: 818 and 747
San Francisco, CA: 415 and 628
This means to get a "local" number you do not need to search for just one area code but can look for a secondary, overlaying code. Wikipedia contains a list of overlayed area codes here.