A telephone numbering plan is a type of numbering scheme used in telecommunications to allocate telephone numbers to subscribers and to route telephone calls in a telephone network. A closed numbering plan, such as found in North America, imposes a fixed total length to numbers. An open numbering plan features variance in the length of telephone numbers.
A dial plan establishes the expected number and pattern of digits for a telephone number. This includes country codes, access codes, area codes and all combinations of digits dialed. For instance, the North American public switched telephone network (PSTN) uses a 10-digit dial plan that includes a 3-digit area code and a 7-digit telephone number. Most PBXs support variable-length dial plans that use 3 to 11 digits. Dial plans must comply with the telephone networks to which they connect.
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In early telephone systems, connections were made in the central office by telephone operators using patch cords to connect one party to another. If a person wanted to make a phone call, he or she would pick up a phone and wind a crank on the side. The crank was a small generator that would light a lamp at the central office. An operator would see the light and insert their patch cord into a socket and assist the customer with the call connection. The operator would use patch cords to connect the caller to the person being called. If the party being called was in another exchange, the operator would use a patch cord to connect to another exchange where an operator elsewhere would finish the connection. As technology advanced, electro-mechanical switches were introduced and calls were made using rotary dials.
Initial use of area codes in the United States began in the 1950s with large cities. By 1966, the system was nationwide.[1]
Area codes were assigned based on the length of time a rotary dial phone took to dial the area code. Densely populated areas like New York City, Chicago, Los Angeles, and Detroit had huge incoming call volume and were assigned numbers (212, 312, 213, 313) that could be quickly dialed from a rotary dial phone. On a rotary dial phone low digits (1, 2, 3, 4) could dial quickly as the time the rotary dial took to return to the home position was minimal. High digit numbers (7, 8, 9, 0) on rotary dial phones took much longer to return to the home position and were usually used in less densely populated areas like rural Texas (915). This numbering strategy became unnecessary when touch-tone phones arrived, as the tone allowed instant entry of digits.
The original set of North American area codes were unique. When the North American Numbering Plan was first 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 be placed was a toll call. Area Code 617 in eastern Massachusetts used this system in the early 1970s, while the Chicago area (Area Code 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 being replaced with ESS machines (Electronic Switching Systems), and the Area Code logic was no longer necessary. The need for more telephone numbers was increasing rapidly, and the NANPA (North American Numbering Plan Administration) was running out of “n0n” and “n1n” combinations. 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 entered (by dial and Touch-Tone key pad), then 10 more digits were expected. If the first digit entered was NOT a “1” only 7 digits were expected and the call would remain within area code. 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.
Currently, because of Area Code overlays, nearly all metropolitan and many rural telephone calls require the full 11 digit entry to complete a telephone call.
Most telephone numbers belong to the E.164 numbering plan, though some PABXs have internal extensions.
The E.164 numbering plan for telephone numbers includes:
Apart from the use of numbering plans for telephone numbers, they are also used in routing of 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 prepended 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 the United States and Canada: "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 Areas (NPAs) and formerly known as STD codes in the UK. These are necessary (for the most part) 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.[2]
The area code must usually be preceded in the dial string by either the national access code ("0" for many countries, "1" in USA and Canada) or the international access code and country code. For non-geographic numbers, as well as mobile telephones outside of the United States and Canada, the "area code" does not correlate to a particular geographic area.
Area codes are often quoted including the national access code, for example a number in London: 020 8765 4321. Users must then correctly interpret the "020" as the code for London. If calling from another number in London they merely dial 8765 4321, or if dialing from another country, drop the "0" and dial: +44 20 8765 4321.
Area codes were introduced in the United States by the Bell System, in 1951, with the first direct dial long distance call.[3]
The local number (or subscriber number) must always be dialed in its entirety. The first few digits in the local number typically indicate smaller geographical areas or individual telephone exchanges. In mobile networks they may indicate a network provider in case the area code does not. Callers from a number with a given area/country code usually do not need to (but optionally may) include the particular area/country code in the number dialed, which enables shorter "dial strings" to be used. Devices that dial phone numbers automatically can include the full number with area and access codes, since there is no additional annoyance related to dialing extra digits.
Although the International Telecommunication Union (ITU) has attempted to promote common standards among nation states, numbering plans take different formats in different parts of the world. For example, the ITU recommends that member states adopt 00 as their international access code. However, as these recommendations are not binding on member states, some have not, such as the United States, Canada, and other countries and territories participating in the North American Numbering Plan.
The international numbering plan establishes country codes, that is, area codes that denote nations or groups of nations. The E.164 standard regulates country codes at the international level and sets a maximum length limit on a full international phone number (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:
Generally the area codes 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 often not the case in the United States where rates to call within a state [regulated by that state's public utilities commission] are often higher than rates to call across the country [generally determined by competition]. In some United States locations where area codes cover more territory, different rates will apply within a single area code depending on the distance to the number called. Rates are determined from rate centers, and rates are set in zones of zero to six miles, six to twelve miles, and so on. This has changed dramatically with the deregulation of local phone services.) Each year, more customers switch to a fixed rate, "all-you-can-dial" plan covering the state, the United States, and/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 have 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 (e.g., Egypt), calls are charged at the same rate regardless of area and in others (e.g., the UK) an area code is occasionally treated as two parts with different rates.
In the U.S., some typical dial plans include:
Many corporate telephone systems use a dialing prefix, typically the digit 9, to obtain a line to the public switched telephone network.
An open dialing plan is one in which there are different dialing arrangements for local and long distance telephone calls. This means that to call another number within the same city or area, callers need dial only the number, but for calls outside the area, an area code is required. In this situation it is customary to show the area code in parentheses, signifying that in some cases the area code is optional or is not required, as suggested by ITU-T Recommendation E.123. The area code is prefixed by a trunk code (usually "0"), which is omitted when calling from outside the country.
To call a number in Sydney, Australia for example:
Note that the "+" is not dialed, it signifies that first the international access code must be dialed, followed by the country code, in this case 61, followed by the number. When using a mobile telephone to place a call, all GSM phones and many models using other technologies do allow the + to be entered and this is internally converted to the correct access code, based on caller location, as the call is made.
New Zealand has a special case of an open dialing 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, to call a phone number in Dunedin:
In the United States, Canada, and other countries or territories using the North American Numbering Plan (NANP), the trunk code is '1', which is also (by coincidence) the country calling code. The same rule also applies in many parts of the NANP, including all areas of Canada that still have an open dialing 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 trunk code (also known as the 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:
To call a number in San Francisco, California the dialing procedure will vary:
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 a 1. In this situation, where the area code is not optional, the area code is not enclosed in parentheses. Thus:
In areas served by AT&T in California, pre-recorded messages instruct customers that use of the local area code within the area code is not permissible and that only the xxx xxxx format is permitted.
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 "feature" 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.
Open and closed dialing plans should not be confused with open and closed numbering plans. A closed numbering plan, such as found in North America, features fixed length area codes and local numbers. An open numbering plan, as found in assorted countries that have not yet standardized, features variance in length of area code or local number, or both. Closed dialing plans are rare where numbering plans are open.
A closed numbering plan is one in which the subscriber's 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.:
In other countries, such as France, Belgium, Switzerland, South Africa and some parts of North America where the dialing 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 dialled, 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 require full national dialing and 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:[5]
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 following syntax used to identify a dial plan in a digit map is adapted from [RFC 2705].
To specify a |
Enter the following |
Result |
|---|---|---|
| Digit |
|
Identifies a specific digit (do not use #) |
| Range |
|
Identifies any digit dialed that is included in the range |
| Range |
|
Specifies a range as a comma separated list |
| Wild card |
|
x matches any single digit that is dialed |
| Wild card |
|
. matches an arbitrary number of digits |
| Timer |
|
Indicates that an additional time out period of 4 seconds should take place before automatic dialing starts |
Some dial plan examples using the above syntax look as follows:
For calls to |
Users dial |
Dial plan |
|---|---|---|
| Internal Extension |
|
xx |
| Local Number |
|
9xxxxxxxT |
| Emergency |
|
911 |
| Local Operators |
|
90T |
| Long Distance |
|
91xxxxxxxxxx |
| International |
|
9011x.T |
The Numbering Plan Indicator (NPI) is a number which is defined in the ITU standard Q.713, paragraph 3.4.2.3.3, indicating the numbering plan of the attached telephone number. NPIs can be found in SCCP and SMS messages. As of 2004[update] the following numbering plans and their respective Numbering Plan Indicator values have been defined:
| NPI | Description | Standard |
|---|---|---|
| 0 | unknown | |
| 1 | ISDN Telephony | E.164 |
| 2 | generic | |
| 3 | data | X.121 |
| 4 | telex | F69 |
| 5 | maritime mobile | E.210 and E.211 |
| 6 | land mobile | E.212 |
| 7 | ISDN/mobile | E.214 |