International Direct Dialing (IDD)

International Direct Dialing lets a telephone user place international calls without operator assistance by dialing an international access prefix, a country calling code, then the domestic number.

Overview

International Direct Dialing (IDD) is the method by which a telephone user places a call to another country directly, without routing the call through a human operator. Over the late twentieth century IDD largely replaced operator-assisted international calls for ordinary subscribers. Today most international voice traffic, and many text and data sessions, are initiated directly by the caller or by automated equipment.

How it works

To make an IDD call, the caller typically dials three parts in sequence: the international access prefix (sometimes called the international call prefix), the country calling code, and the national telephone number for the destination. Modern mobile handsets accept the plus sign (+) as a substitute for the access prefix; pressing and holding the zero key frequently generates this sign.

For example, a generic dialing sequence looks like this: international access prefix + country calling code + national significant number. When numbers are published for an international audience they are commonly written with a leading "+" followed by the country calling code and the rest of the number; this signals the caller to use the correct local access prefix.

Key components

International access prefix: the digits dialed to begin an international call from the originating country (for many countries this is "00", while the North American Numbering Plan uses "011"; other countries use different codes).
Country calling code: the numeric code assigned by the International Telecommunication Union (ITU) that identifies the destination country or service area.
National significant number: the domestic portion of the telephone number, which may include area or city codes and the subscriber number. Leading domestic trunk prefixes (often a zero) are normally omitted when dialing from abroad.

History and standards

The technical framework for global telephone numbering and addressing is coordinated by the ITU. The ITU’s E.164 recommendation defines the international public telecommunication numbering plan, which specifies maximum lengths and the structure needed to route calls worldwide. The move from operator-mediated international connections to direct dialing occurred gradually during the mid- to late twentieth century as switching systems, signaling technologies and international agreements matured.

Uses, evolution and notable facts

IDD remains the primary method for placing voice calls between countries. However, the landscape has evolved: mobile phones accept the "+" notation, Voice over IP (VoIP) services can route calls over the internet and often present numbers in international form, and some special service numbers are non-geographic. Published telephone numbers intended for international callers are formatted with the country code and omit local trunk prefixes to avoid confusion.

Costs, quality and routing can vary: international calls may be charged at different rates, use different transit carriers, or be routed via multiple networks. Emergency numbers and local short codes are handled differently and are not reached via IDD. For travelers, dialing conventions differ by country, so the "+" convention and awareness of the local international access prefix simplify placing calls while abroad.