||The examples and perspective in this article may not represent a worldwide view of the subject. (August 2010)|
A telephone numbering plan is a type of numbering scheme used in telecommunications to assign telephone numbers to subscriber stations and to connect telephone calls in a telephone network using the method of destination code routing.
A closed numbering plan, such as found in North America, imposes a fixed number of digits to telephone numbers, while an open numbering plan allows variance in the numbers of digits.
The North American Numbering Plan of the public switched telephone network (PSTN) prescribes two basic parts of its destination routing codes, which are fixed-length strings of ten digits. The first part is a 3-digit area or Numbering Plan Area (NPA) code, and the second part is a 7-digit telephone number. The telephone number consists of a 3-digit central office (CO) code followed by a 4-digit station number.
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. Such numbering strategies became unnecessary when the Bell System introduced Dual-tone multi-frequency (DTMF) signaling in its Touch-Tone® service starting in 1963, in which all digits were generated in the same amount of time.
|This section does not cite any references or sources. (August 2010)|
When the North American Numbering Plan was developed, logic circuits consisted of relays and vacuum tubes, and simplicity was mandatory. The second digit of all area codes was 0 or 1, while the second digit of the exchange triplet was never 0 or 1, thus facilitating readily the recognition whether the dialing of a full 10-digit number was in progress or whether the user was dialing only a local number. In this coding scheme, a leading 1 indicating long distance, or out of the area code call, was not necessary. In some regions, local numbers were 7 digits (occasionally less), but a toll call within the area code required a preceding "1". This told the subscriber that the call being placed was a toll call. Area code 617 in eastern Massachusetts used this system in the early 1970s, while the Chicago area (312) did not. In the Chicago area, one could call the Boston area by dialing only 10 digits, while in Boston, to call Chicago, one would be required to use 11 digits (preceding the 10 digits with a "1")
By the 1990s, the mechanical central office switches were rapidly replaced with Electronic Switching System (ESS) equipment, and the area code logic was no longer necessary. The demand for more telephone numbers was increasing rapidly, and the possible "n0n" and "n1n" combinations were insufficient to sustain growth. This area code scheme was abandoned, with the result that former area code only numbers could now be exchanges, and former exchange only numbers could now be area codes. This caused a logic dilemma: The switch had no way of knowing whether to expect 10 digits or 7 digits. The solution was simple. If a preceding "1" was dialed then 10 more digits were expected. If the first digit entered was not a "1", only 7 digits were expected and the area code was inferred from the originating subscriber number. For a short while, in some area codes, one could enter the full 11 digits for a call within their own neighborhood or just enter the last 7 digits, and the call would be routed and billed identically.
Because of the popularity of fax machines and cell phones, far more telephone numbers were needed to handle the demand, and the decision was taken to add more area codes rather than increase the number length. Since 1995, there have been over 380 new area codes added in the North American Numbering Plan area. Some created area code splits (where an existing area code was split up into two or more new area codes). This cost businesses and others a lot of money reprinting stationery, business cards, etc. These were often greatly contested as to which area could keep the old area code (usually the largest city kept the old code). For instance, 305 was split in 1995. It used to serve both the Miami and Fort Lauderdale area. Dade County (at the time, now called Miami-Dade) kept 305 and Broward County (the Fort Lauderdale area) had to change to 954. Because splits were challenged so strongly, overlays were created when possible. An overlay was a new area code that covered the same geographical area as an existing code. This enabled people to keep their old area code, while new phone numbers would get the new area code. There have been over 75 overlays introduced since 1995.
Currently, because of area code overlays, nearly all metropolitan and many rural telephone calls require the full 10 digit entry to complete a telephone call.
Apart from the use of numbering plans for telephone numbers, they are also used in routing of Signaling System 7 (SS7) signalling messages as part of the Global Title. In public land mobile networks, the E.212 numbering plan is used for subscriber identities (e.g., stored in the GSM SIM) while E.214 is used for routing database queries across PSTN networks.
Country code - necessary only when dialing to phones in other countries. In international usage, telephone numbers are prefixed with the country code preceded by a "+", and with spaces in place of hyphens (e.g., "+XX YYY ZZZ ZZZZ"). This allows the reader to choose which Access Code (also known as International Dialing Digit) they need to dial from their location. However, it is often quoted together with the international access code which must precede it in the dial string, for example "011" in NANP countries (including Canada, Bermuda, and the United States): "011-XX-YYY-ZZZ-ZZZZ", or "00" in most European countries: "00-XX-YYY-ZZZ-ZZZZ". This can cause confusion as a different Access Code may be used where the reader is located. On GSM networks, "+" is an actual character that may be used internally as the international access code, rather than simply being a convention.
Area codes are also known as Numbering Plan Area (NPA) codes and formerly known as subscriber trunk dialling (STD) codes in the UK. These are typically necessary only when dialed from outside the code area, from mobile phones, and, especially within North America, from within overlay plans. Area codes usually indicate geographical areas within one country that are covered by perhaps hundreds of telephone exchanges, although the correlation to geographical area is becoming obsolete.
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) or the international access code and country code. However, this is not always the case, especially when 10-digit dialing is used. For example, in Montreal, where area codes 514, 438, 450 and 579 are in use, users dial 10-digit numbers (e.g., 514 555 1234), dialing a 1 before this results in a recording advising not to dial a 1 as it is a local call. For non-geographic numbers, as well as mobile telephones outside of the North American Numbering Plan area, the area code does not correlate to a particular geographic area. However, until the 1990s, some areas in the United States required the use of a 1 before dialing a 7-digit number within the same area code if the call was beyond the local toll-free area, indicating that the caller wished to make what was referred to as a toll call.
Area codes are often quoted including the national access code, for example a number in London: 020 8765 4321. Users must correctly interpret the 020 as the code for London. If they call from another number in London, they merely dial 8765 4321, or if dialing from another country, the 0 must be dropped and the country code added: +44 20 8765 4321.
When the Bell System first designed these codes in the 1940s, the initial format was NBX, where N could be any digit from 2 to 9, B either 0 or 1, and X any digit. Since all telephones in this era used rotary mechanisms, the Bell System wanted to reduce the time required for dialing during which a circuit had to be dedicated, as well as lower the overall mechanical wear by reducing the number of clicks on numbers that would be dialed most often. Areas with higher call volumes would be assigned lower first and third digits and a 1 as the center digit. The digit 0 was located at the end of the dial after 9, making zero the longest digit to dial. New York, having the highest call volume, was assigned the 212 area code, the shortest code that could be dialed. This was followed by Los Angeles at 213, and Chicago at 312. The next code, 313, did not go to Philadelphia, the next largest in population at the time. Instead the code went to Detroit since it had a higher call volume. Philadelphia received the 215 area code instead, while the Dallas–Fort Worth area received 214, even though Dallas–Fort Worth was less-populated at the time.
In 1995, the center digit rule was relaxed, allowing it to be any digit except 9. 9 as the middle digit of an NPA is reserved in case 3-digit area codes run out and have to be augmented to 4 digits.
The subscriber number is the local number assigned to the line connected to a customer's equipment. It must always be dialed in its entirety. The first few digits of the subscriber number typically indicate smaller geographical areas or individual telephone exchanges. In mobile networks they may indicate the network provider. Callers in a given area or country usually do not need to include the particular area prefixes when dialing within the same area. Devices that dial telephone numbers automatically may include the full number with area and access codes.
Although the International Telecommunication Union (ITU) has attempted to promote common standards among member states, numbering plan administrations use different number formats in different parts of the world. For example, the ITU recommends that member states adopt the prefix 00 for international access. However, these recommendations are not binding, and some countries use other codes, such as the United States, Canada, and other countries and territories participating in the North American Numbering Plan, where the international trunk prefix is 011.
The E.164 standard is a international numbering plan and establishes country codes, which are prefixes that denote nations or groups of nations. It sets a maximum length limit on a full international phone number to 15 digits. However, it is each country's responsibility to define the numbering within its own network. As a result, regional area codes may be:
In countries other than the United States and Canada, the area codes generally determine the cost of a call. Calls within an area code and often a small group of adjacent or overlapping area codes are normally charged at a lower rate than outside the area code. This is not necessarily the case in the United States or Canada where rates are determined by the distance between rate centers. In most United States and Canadian locations area codes cover a sufficiently large territory that different rates will apply within the same area code determined by the distance between rate centers. Each rate center also has a local calling plan which determines which other rate centers, regardless of distance, are a local call. The area code plus the first three digits following the area code (the NPA-NXX) defines the rate center for any given telephone number. Multiple NPA-NXX combinations, often in different area codes, comprise each rate center, and each rate center is assigned a geographic point known as the rate center's V&H coordinates. The tariff distance in miles between any two rate centers may be calculated using by this formula: .
Calls between nearby rate centers in different area codes may be cheaper (or even free local calls) as compared to calls to more distant rate centers in the same area code. Rates are set in zones of 0-6 mi, 6-12 mi, and so on, with these bands determined on a state-by-state basis for intrastate calls (calls within the same state) and determined by federal regulation for interstate calls (calls which cross a state line). As a specific example, callers in the Falls Church, Virginia, rate center (officially named "Washington Zone 17, VA"—example numbers beginning with 703-534, V=5636, H=1600) may make untimed local calls to 31 other nearby rate centers in Virginia, Maryland, and the District of Columbia in area codes 703, 571, 202, 301, and 240, while calls to distant locations in 703, such as Manassas and Haymarket, VA, are charged as long distance.
Calls within a state [regulated by that state's public utilities commission] are often higher than rates to call more distant locations in some other state [regulated by the Federal Communications Commission]. The partial deregulation and introduction of competition for long-distance phone services has established other methods of determining call pricing that do not necessarily follow the traditional model. Each year, more customers switch to a fixed rate, "all-you-can-dial" plan covering the state, the United States, or all North America generally (as of May 2008 and exclusive of taxes) for approximately $30 per month. Competition with cable telephony and Voice over Internet Protocol services has helped drive the cost of service down for residential and business customers.
Special area codes are generally used for free, premium-rate, mobile phone systems (in countries where the mobile phone system is "caller pays") and other special-rate numbers. There are, however, some exceptions: in some countries (like Egypt), calls are charged at the same rate regardless of area and in others (like the UK) an area code is occasionally treated as two areas with different rates.
A dial plan establishes the expected sequence of digits dialed on subscriber premise equipment, such as telephones, in private branch exchange (PBX) systems, or in other telephone switches to effect access to the telephone networks for the routing of telephone calls, or to effect or activate specific service features by the local telephone company, such as 311 or 411 service.
A variety of dial plans may exist within a numbering plans, whether it is closed or open, and these often depend on the network architecture of the local telephone operating company.
Within the North American Numbering Plan, the administration defines standard and permissive dialing plans, specifying the number of mandatory digits to be dialed for local calls within the area code, as well as alternate, optional sequences, such as adding the trunk code 1 before the telephone number.
Despite a closed numbering plan, different dialing procedures exist in many of the territories for local and long distance telephone calls. This means that to call another number within the same city or area, callers need to dial only a subset of the full telephone number. For example in the NANP, only the 7-digit number may need to be dialed, but for calls outside the area, the full number including the area code is required. In these situations, the ITU-T Recommendation E.123 suggests to list the area code in parentheses, signifying that in some cases the area code is optional or is not required. Typically the area code is prefixed by a domestic trunk access code (usually 0) when dialing from inside a country, but is not necessary when calling from other countries, but there are exceptions, such as for Italian land lines.
To call a number in Sydney, Australia for example:
The plus character (+) in the markup is not dialed, it signifies that the following digits are the country code, in this case 61, and that an international access code is required. Mobile telephone service using GSM and other technologies allow the + to be entered and this is internally converted to the correct access code, based on caller location, as the call is placed.
New Zealand has a special case dial plan. While most nations require the area code to be dialed only if it is different, in New Zealand, one needs to dial the area code if the phone is outside the local calling area. For example, the town of Waikouaiti is in the Dunedin City Council jurisdiction, and has phone numbers (03) 465 7xxx. To call the city council in central Dunedin (03) 477 4000, residents must dial the number in full including the area code even though the area code is the same, as Waikouaiti and Dunedin lie in different local calling areas (Palmerston and Dunedin respectively)
In the United States, Canada, and other countries or territories using the North American Numbering Plan (NANP), the international trunk access code is 1, which is also the country calling code. The same rule also applies in many parts of the NANP, including all areas of Canada that still have variable-length dial plan. This is not universal, as there are locations within the United States that allow long distance calls within the same area code to be dialed as seven digits. In Canada, the domestic trunk code (long distance access code) must also be dialed along with the area code for long distance calls even within the same area code. For example, to call a number in Regina:
However, in parts of North America, especially where a new area code overlays an older area code, dialing the area code, or 1 + the area code, is now required even for local calls, which means that the NANP is now closed in certain areas and open in others. Dialing from mobile phones is different in the U.S., as the trunk code is not necessary, although it is still necessary for calling all long distance numbers from a mobile phone in Canada. (Most mobile phones today can be programmed to automatically add a frequently-called area code as a prefix, allowing calls within the desired area to be dialed by the user as seven-digit numbers, though sent by the phone as 10-digit numbers.)
In some parts of the United States, especially northeastern states such as Pennsylvania served by Verizon Communications, the full 10-digit number must be dialed. If the call is not local, the call will not complete unless the dialed number is preceded by digit 1. In this situation, where the area code is not optional, the area code is not enclosed in parentheses. Thus:
In California, because of the existence of both overlay area codes (where an area code must be dialed for every call) and non-overlay area codes (where an area code is dialed only for calls outside the subscribers home area code), "permissive home area code dialing" of 1 + the area code within the same area code, even if no area code is required, has been permitted since the mid-2000s (decade). For example, in the 213 area code (a non-overlay area code), calls may be dialed as 7 digits (XXX-XXXX) or 1-213 + 7 digits. The manner in which a call is dialed does not affect the billing of the call. This "permissive home area code dialing" helps maintain uniformity and eliminates confusion given the different types of area code relief that has made California the nation's most "area code" intensive State. Unlike other states with overlay area codes (Texas, Maryland, Florida and Pennsylvania and others), the California Public Utilities Commission maintains two different dial plans: Landlines must dial 1 + area code whenever an Area Code is part of the dialed digits while Cell Phone can omit the "1" and just dial 10 digits.
Many organizations have private branch exchange systems which permit dialing the access digit(s) for an outside line (usually 9 or 8), a "1" and finally the local area code and xxx xxxx in areas without overlays. This aspect is unintentionally helpful for employees who reside in one area code and work in an area code with one, two, or three adjacent area codes. 1+ dialing to any area code by an employee can be done quickly, with all exceptions processed by the private branch exchange and passed onto the public switched telephone network.
In small countries or areas, the full telephone number is used for all calls, even in the same area. This has traditionally been the case in small countries and territories where area codes have not been required. However, there has been a trend in many countries towards making all numbers a standard length, and incorporating the area code into the subscriber's number. This usually makes the use of a trunk code obsolete. For example, to call Oslo in Norway before 1992, one would dial:
After 1992, this changed to a closed eight-digit numbering plan, e.g.:
Therefore in other countries, such as France, Belgium, Japan, Switzerland, South Africa and some parts of North America where the numbering plan is closed, the trunk code is retained for domestic calls, whether local or national, e.g.,
while some, like Italy, require the initial zero to be dialed, even for calls from outside the country, e.g.,
Further, there are locations with closed dialing plans in the NANP that require the full phone number including area code to be dialed for all calls, but the trunk code is required for only long distance calls, even in the same area code.
While the use of full national dialing is less user-friendly than using only a local number without the area code, the increased use of mobile phones, which can store numbers, means that this is of decreasing importance. It also makes easier to display numbers in the international format, as no trunk code is required—hence a number in Prague, Czech Republic, can now be displayed as:
as opposed to before September 21, 2002:
Some countries already switched, but trunk prefix re-added with the closed dialing plan, for example in Bangkok, Thailand before 1997:
has been switched in 1997:
Trunk prefix has re-added in 2001
Satellite phones are usually issued with numbers in a special country calling code. For example, Inmarsat satellite phones are issued with code +870, while Global Mobile Satellite System providers, such as Iridium, issue numbers in country code +881 ("Global Mobile Satellite System") or +882 ("International Networks"). Some satellite phones are issued with ordinary phone numbers, such as Globalstar satellite phones issued with NANP telephone numbers.
Some country calling codes are issued for special services, or for international/inter regional zones.
The numbering plan indicator (NPI) is a number which is defined in the ITU standard Q.713, paragraph 184.108.40.206.3, indicating the numbering plan of the attached telephone number. NPIs can be found in Signalling Connection Control Part (SCCP) and short message service (SMS) messages. As of 2004[update], the following numbering plans and their respective numbering plan indicator values have been defined:
|5||maritime mobile||E.210 and E.211|
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