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Notation for Telephone Numbers:

International Number Format is specified by the International Telecommunication Union - Telecom Standardization Committee (ITU-T) Study Group 13 [GII (Global Information Infrastructure) ergonomics issues] in: ITU-T Recommendation E.123
Notation for national and international telephone numbers, e-mail addresses and Web addresses
E.123 pdf

E.g. for Geneva (Area/City/Trunk Code = 22) Switzerland (Country Code=41): 

Telephone: National       (703) 234 5678
           International +1 703 234 5678
 E-mail:   johndoe@xyz.com
 Web:      www.itu.int
     (22) 730 5989 ...... For Numbers in the same country or num. plan area.
  +41 22 730 5989 ....... For International numbers.
In the above examples: 22 is the area-city code, called the "trunk code (TC)" or "National destination code (NDC)" by CCITT. The space following the area-city code should be wider (Optn.-Space).
41 is the Country Code. + represents the international access code or prefix, e.g., (011) when calling from the USA.
The last part of the number is called the subscriber number.
(parentheses) indicate digits that are not always dialed.
Calls to numbers in the same local calling area may not require the city or area code.
Calls outside the local area require a trunk prefix, e.g., 1 in the USA and an area or city code.
Note: Some countries may show telephone numbers with the trunk prefix as part of the area or city code, e.g., (022).
The trunk prefix "0" should not be included for international dialing.
Grouping of digits within a number should be accomplished with spaces rather than dots or hyphens.
To show an extension number of a PABX without direct in-dialing, the nationally used word or abbrevitaion for "extension" should be used. e.g.(302) 123 4567 ext. 876 When printing telephone numbers on letterheads, directories, etc. 
both National and International format should be shown as follows:
           Within N. Amer. zone   (302) 123 4567
  Telephone  ______________________________
              International     +1 302  123 4567


                         Area/
                  Country City (1)         Sample Listing    (2)
Location            Code  Code IDD  Intl. Format         Local Format
Geneva, Switzerland  41    22   00  Tel: +41 22 730 5989 (022) 730 5989
                                    or (+41) 22 730 5989
Washington DC, USA    1   202  011  +1 202 418 0190      (202) 418-0190

Dialing Wash DC (the FCC) from Switzerland: 00 1 202 418 0190
Dialing Geneva (the ITU) from the US:  011 41 22 730 5989

(1) International Direct Dialing (IDD) Access Code is the code you dial
    before making an international call.
(2) Local format may include a leading "0" which must be omitted for
 international dialing.
North American Number format is specified by the North American Numbering Plan (NANP)
(NXX) NXX XXXX - fixed-length 10-digit national numbers: 3-digit area codes followed by 7-digit local numbers.

ITU-T Recommendation ITU-T E.164 allows up to 15 digits in an international number including the country code.

World Telephone Numbering Guide . International Numbering Plans at numberingplans.com North American Numbering Plan (NANP)

Expansion of International Number Lengths at ITU

Country Code Lists as specified in ITU-T E.164 at:

numberingplans.com, ACR, Defense Information Systems Agency (DISA)
Mobil Country Codes at: DISA,

ITU-T Recommendation E.123 (02/2001)
Notation for national and international telephone numbers, e-mail addresses and Web addresses
Article: 20897 (English), 20898 (French), 20899 (Spanish)
Price: Paper - CHF 12 (Swiss Francs), Download CHF 20 ($15 USD)

It provides a standard for printing e-mail and website address information on business cards and letterheads.
Draft copy at ngi.org

ITU-T Recommendation E.164
The international public telecommunication numbering plan.
E.164 (05/97) The international public telecommunication numbering plan. Article 11877
E.164 Supplement 1 (03/98) Alternatives for carrier selection and network identification
E.164 Supplement 2 (11/98) Number Portability
E.164 Supplement 3 (05/02) Supplement 3: Operational and administrative issues associated with national implementations of the ENUM functions
E.164.1 (03/98) Criteria and procedures for the reservation, assignment and reclamation of E.164 country codes and associated Identification Codes (ICs)
E.164.2 (02/01) E.164 numbering resources for trials
E.164.3 (09/01) Principles, criteria and procedures for the assignment and reclamation of E.164 country codes and associated identification codes for groups of countries

E.129, Presentation of national numbering plans

The objective of this recommendation is to specify a methodology that will provide a standardized method for presenting the domestic numbering plans of all countries (i.e., each country's application of E.164). This recommendation also includes a method by which this information is made available to all interested parties as well as timely information on numbering plan changes that influence the routing, charging, and accounting of international telecommunications traffic.

E.168, Application of E.164 for UPT

E.212 The international identification plan for mobile terminals and mobile users

E.121 Symbols

F.850 Universal Personal Telecommunications (UPT) service
A telecommunications service that provides personal mobility and service profile management.

Links:
How to write a phone number

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last updated 29 Jan 2003