In early telephone systems,connections were made in the central office bytelephone operators usingpatch 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 usingrotary dials.

Initial use of area codes in the United States began in the 1950s with large cities. By 1966,the system was nationwide.

Area codes were assigned based on the length of time arotary 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 whentouch-tone phones arrived,as the tone allowed instant entry of digits.

Most telephone numbers belong to theE.164 numbering plan,though somePABXs have internal extensions.

TheE.164 numbering plan fortelephone numbers includes:

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Country code

Country code - necessary only when dialing to phones in other countries. In international usage,telephone numbers are quoted 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,especially in the United States and Canada (e.g.,"011-XX-YYY-ZZZ-ZZZZ"). This can cause confusion as "011" may not be a valid Access Code where the reader is located. OnGSM networks,"+" is an actual character that may be used internally as the international access code,rather than simply being a convention.

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Area code

Area codes are also known as Numbering Plan Areas (NPAs). These are necessary (for the most part) only when dialed from outside the code area,from mobile phones,and (especially within North America) from withinoverlay 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 - It must usually be preceded in the dial string by either the national access code or the international access code and country code. For non-geographical 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 were introduced in the United States by theBell System,in 1951,with the first direct dial long distance call.[ 3 -

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Local number

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 individualtelephone 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 theInternational 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 theUnited States,Canada,and other countries and territories participating in theNorth American Numbering Plan.

The international numbering plan establishescountry codes,that is,area codes that denote nations or groups of nations. TheE.164 standard regulatescountry 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:

Special area codes are generally used forfree,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.,theUK) an area code is occasionally treated as two parts with different rates.

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Example

In theU.S.,some typical dial plans include:

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Open dialing plans

An opendialing plan is one in which there are different dialing arrangements for local andlong 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 byITU-T RecommendationE.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:

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 inDunedin :

In some parts of the United States,especially northeastern states such asPennsylvania served byVerizon 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:

Many organizations haveprivate 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 thepublic 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.

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Closed dialing plans

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 inPrague,Czech Republic,can now be displayed as:

Satellite phones are usually issued with numbers in a special country calling code. For example,Inmarsat satellite phones are issued with code +870,whileGlobal Mobile Satellite System providers,such asIridium,issue numbers in country code +881 ("Global Mobile Satellite System") or +882 ("International Networks"). Some satellite phones are issued with ordinary phone numbers,such asGlobalstar satellite phones issued with NANP telephone numbers.

Somecountry 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]^{page needed ]}.

The Numbering Plan Indicator (NPI) is a number which is defined in the ITU standardQ.713,paragraph 3.4.2.3.3,indicating the numbering plan of the attached telephone number. NPIs can be found inSCCP andSMS messages. As of 2004^{update - the following numbering plans and their respective Numbering Plan Indicator values have been defined:}