When telephone numbers were first used they were very short, from one to three digits, and were communicated orally to a switchboard operator when initiating a call. As telephone systems have grown and interconnected to encompass worldwide communication, telephone numbers have become longer. In addition to telephones, they have been used to access other devices, such as computer modems, pagers, and fax machines. With landlines, modems and pagers falling out of use in favor of all-digital always- not connected broadband Internet and mobile phones, telephone numbers are now instead taken by data-only cellular devices, such as some tablet computers, digital television, and even video game controllers and mobile hotspots, on which it is not even possible to make or accept a computer call.

Of all states or territories, the U.S. state of California has the largest number of area codes assigned, followed by Texas, Florida and New York, while most countries of the Caribbean only use one. Many Caribbean codes were assigned based on alphabetic abbreviations of the territory name, as indicated in the third column of the following table.

In the late 1870s, the Bell interests started utilizing their patent with a rental scheme, in which they would rent their instruments to individual users who would contract with other suppliers to connect them; for example from home to office to factory. Western Union and the Bell company both soon realized that a subscription service would be more profitable, with the invention of the telephone switchboard or central office. Such an office was staffed by an operator who connected the calls by personal names. Some have argued that use of the telephone altered the physical layout of American cities

The NANP divides the territories of its members into numbering plan areas which are encoded numerically with a three-digit telephone number prefix, commonly called the area code. Each telephone is assigned a seven-digit telephone number unique only within its respective plan area consisting of a three-digit central office code and a four-digit station number.

Until the 1960s, the North American Numbering Plan used telephone number formats that included the name of the central office to which each telephone was connected. Traditionally, these names were often town names, village names, or other locally significant names, but larger communities that required more than one central office, may have used other names for each central office, such as Main, East, West, Central. Names were convenient to use and reduced errors when telephone numbers were exchanged verbally between subscribers and the operator.

North American Numbering Plan is a telephone numbering plan that encompasses 25 distinct regions in twenty countries primarily in North America, including the Caribbean and the U.S. territories. Not all North American countries participate in the NANP. Each participating country forms a regulatory authority that has plenary control over local numbering resources.

The goal of the North American Numbering Plan was a system by which telephone subscribers in the United States and Canada could themselves dial and establish a telephone call to any other subscriber without the assistance of switchboard operators. For this, each central office in the served territories was assigned a unique designation of similar form to all others.