To search by area code, simply type a 3-digit area code into the Search Text box and click on Search.
To search by city, click on the By City option button, then type the name in the Search Text box and click the Search button. You can type part of a city name; Cincin will retrieve all the area codes in Cincinatti, for instance. Note that this system only contains one major city for each area code. (Though some cities may have multiple area codes.) Thus searching for Denver will return 303, though searching for Boulder (a smaller city in the same area code) will not return anything.
To search by state-province-country, click on the By State-Province-Country option button, type the name of a state (USA), province (Canada), or nation (Caribean) into the Search Text box. Then click on the Search button. You can type part of a name; Calif will retrieve all the area codes in California, for example.
You can also display the entire area-code list, sorted alphabetically by state-province-country. Click on the Display ALL Area Codes option button. A message box opens warning you that it will take a few seconds for this list to appear. Click on OK and the program will, after a few seconds delay, load the list into the large text area at the bottom of the page.
For more information about the North American Numbering Plan, visit Bellcore's site, http://www.bellcore.com/NANP/.