How does the breakdown of area codes work from city to city state to state?

In the United States, telephone area codes do not cross state lines. However, within a state, there are many cases where a telephone area code boundary divides a city or town between two or more area codes.

The only states left that have only one area code each are Alaska, Hawaii, Idaho, Montana, Wyoming, North Dakota, South Dakota, Delaware, Rhode Island, Vermont, New Hampshire, and Maine. In addition, the entire state of West Virginia is an overlay; although there are two area codes, there is no boundary between them, since they both serve the entire state.

The first area code overlay in the United States was area code 917 for New York City. In the original plan, 917 was to be the area code for mobile phones and pagers, with mobile users in the 212 and 718 area codes (then the only two area codes for New York City) forced to move their numbers to 917. However, federal regulators held that it was illegal to discriminate on the basis of class of service (i.e., landline versus cellphone, etc.) in deciding who got the desirable 212 and 718 numbers and who got the less-desirable 917 numbers. The next overlays in the United States were in Maryland in 1997. However, since 2007, there have been no area code splits, only overlays. Canada has had only overlays since 1999. Thus, there are more and more parts of the North American Numbering Plan (the area code system that serves the USA, Canada, and 18 assorted island nations and territories, mostly in the Caribbean) that have two or more area codes for the same location.