I found a great website that will let you lookup a phone number and find out what other area codes and prefixes that number can connect to as a local call. It has really good searching and sorting; you can find a number by city and state or area code and prefix. And once you find an area code and prefix you can see what other prefixes it is able to dial in its local calling area.

The reason I was searching for a prefix is because I recently signed up for another service called Grand Central. Grand Central is an amazing free service by Google that will give you a phone number of your choosing and then allow to route calls for that number to any amount of other phones. It also can screen calls, take messages and even pretend to be a disconnected line to salesmen. Right now Grand Central is invitation only and it looks like they are building in ways to charge for parts of the service, so it remains to be seen if it will be free forever. But it is a great service for me because now I can provide my family a local phone number to reach me at (hence the reason I was looking to see which prefixes were local to each other).

The local calling website.
http://www.localcallingguide.com

Grand Central's website.
http://www.grandcentral.com