Like the QWERTY keyboard design, which was originally designed so as to slow down typing and prevent key jams in the original typewriters; US Area codes also depend on the design of the then rotary telephones. On the rotary telephone dial, the digits are arranged in a circular layout so that a finger wheel may be rotated with one finger from the position of each digit to a fixed stop position.
The area code system, also called the North American Numbering Plan (NANP), was developed by AT&T and Bell Laboratories and was operational in 1947. The details can be found at the NANP Wikipedia page. What is interesting is that the first and third digits of the three digit code were allotted according to the population density of the city or region – the most populated areas getting the lowest numbers. The New York City area, for example, was assigned 212, Washington DC. 202, Pennsylvania 814. The rationale for this scheme was that with rotary dials in those days lower numbers resulted in shorter “dial pulls” so as to produce less wear on the dialer mechanism and also to provide “less work” to the people dialing – or so the reasoning goes.
In short you could reasonably guess the population density of a city or area by looking at the corresponding area code.