An Area code is a three-digit number that identifies one of the telephone service regions into which the US, Canada, and certain other countries are divided and that is dialed when calling from one area to another. Area codes are also known as Numbering Plan Area (NPA) codes and formerly known as subscriber trunk dialling (STD) codes. The area code is usually preceded in the dialing sequence by either the national access code (“0” for many countries, “1” in USA) and Canada) followed by the area code and then the local number (or subscriber number).
To call a number in San Francisco, California the dialing procedure will vary:
xxx xxxx (local or long-distance within area code 415, no area code required)
1 (415) xxx xxxx (local or long-distance outside of 415 but within the U.S., Canada, and other countries in the NANP)
+1 (415) xxx xxxx (outside the NANP – 1 is the country code for the U.S.)
Phone Number Area Code Guide
In early telephone systems, connections were made in the central office by switchboard operators using patch cords to connect one party to another. To make a telephone call, a person would wind a crank to generate a ring signal to the central office operator, either before or after the user took the telephone handset off-hook. At the central office a gong or later an electric light indicated the need to respond to the customer, upon which the operator inserted a patch cord into a socket and assisted the customer with the call by voice. Another patch cord connected the caller to destination telephone line. If the destination party belonged to another exchange, the operator used a patch cord to connect to that exchange where an operator would complete the call setup. As technology advanced, automatic electro-mechanical switches were introduced and telephones were equipped with initially rotary dials for pulse-dialing, and in the 1960s with Touch-Tone key pads which increased the speed of dialing and enabled other vertical telephone features.
Initial use of area codes in the United States and Canada began in 1947 in large cities. By 1966, the system was implemented fully in both countries. Area codes were initially assigned based on the volume of telephone calls made in each area. The most populous areas were assigned the area codes that required the least time for dialing using a rotary dial telephone. A subscriber line as well as switch banks had to be allocated for the entire time from begin of dialing to the end of a call. The time to dial a digit was directly proportional to the digit, with the exception of the 0, which required 10 pulses or 1 second. The densely populated areas of New York City, Chicago, Los Angeles, and Detroit had large incoming call volume and were assigned the area codes (212, 312, 213, and 313, respectively. As an exception, the first area code installed was 201 for New Jersey. The sparsely populated area of rural Texas received area code 915. This numbering strategy became unnecessary when Dual-tone multi-frequency (DTMF) signaling was used in Touch-Tone® telephones starting in 1963, in which all digits could be generated in the same amount of time.