North American Numbering Plan
The North American Numbering Plan is a closed numbering plan, which prescribes ten digits for each complete destination routing code, including a three-digit area code followed by a three-digit exchange and then four more digits. In the United States and Canada, area codes were first allocated in 1947. In some large cities they were used soon after as operator routing codes for connecting long-distance telephone calls between toll switching centers. The first customer-dialed long-distance calls were possible in Englewood, NJ in 1951. By ca. 1966, the system was implemented fully in both countries and users of the telephone system needed to learn their own area code. The Bell System organized the numbering plan to minimize the cost of providing automatic dialing to large population centers, as calls that crossed area code boundaries were required to be switched by special toll switching systems. For example, geographic areas were designed so that tributary routes were placed into the same area as the major toll center.
Due to a technical limitation of the numbering plan , states that were anticipated to require more than about 500 central offices were split into multiple areas, each receiving a code with the middle digit being 1. On the other hand, area codes that covered an entire state had the digit 0 in the middle. In contrast to the area code, the second digit of the three-digit exchange code was never 0 or 1, thus affording a simple rule for recognition of whether a user was dialing a full ten-digit telephone number or merely dialing within the local area code using seven-digit dialing. Toll operators were able to differentiate between the two types of areas from the middle digit of the area code when a routing operator had to be consulted.
By the 1990s, most electromechanical central office switches had been replaced with electronic switching system (ESS) equipment and the previous area code logic was no longer necessary. The demand for telephone numbers was increasing rapidly, and the remaining n0n and n1n combinations were insufficient to sustain growth. In response, this limitation was abandoned for new codes, with the result that area codes and central office codes could no longer be immediately distinguished by the switching equipment.
The solution was to require the dialing of a preceding 1 for any call between different area codes, in which case the equipment expects 10 more digits; if the first digit dialed is not a "1", only 7 total digits are expected, and the area code is inferred from the originating subscriber's area code. (This 1+10-digit dial pattern had previously been used in some areas to distinguish "local" and "toll" calls.) In many areas assigned only one area code, one can thus enter either the full 11 digits for a call within their own code, or just enter the last 7 digits, and the call is routed and billed identically. When an additional (overlay) code is assigned, generally 1+10 digit dialing becomes mandatory. In some areas, this led to early resistance to overlay codes,
but this has usually been outweighed by the preference to avoid number changes for existing subscribers.
The rising popularity of fax machines and pagers required far more telephone numbers than were anticipated in the design of the numbering system. As a remedy, the restrictions on the format of area codes were eased. Since 1995, over 380 new area codes were added to the North American Numbering Plan. Some areas used area code splits, by which an existing numbering plan area (NPA) was split into multiple divisions each assigned a new area code. Thus, many businesses were required to reprint business stationery, catalogs, and directories. Area code splits were often contested as to which area could keep the existing code, which usually fell to the largest city. For example, 305 served both the Miami and Fort Lauderdale areas until 1995. Dade County (now Miami-Dade) kept 305 and Broward County (Fort Lauderdale area) changed to 954. Another method was using area code overlays, which avoided renumbering existing stations. An overlay is a new area code that covers the same geographical area as an existing code. Over 75 overlays have been introduced since 1995. Area code overlays invariably require ten-digit dialing (without the 1+ long distance prefix) of telephone numbers in the numbering plan area. Internet telephony services are not tied to physical locations and area codes often no longer correspond to the physical location of the provider, nor the subscriber.