International direct dialing

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International direct dialing (IDD) or international subscriber dialling (ISD) is placing an international telephone call, dialed directly by a telephone subscriber, rather than by a telephone operator. Subscriber dialing of international calls typically requires an international call prefix (international dial-out code, international direct dial code, IDD code) to be dialed before the country code.

The term international subscriber dialling was used in the United Kingdom and Australia until the terminology was changed to international direct dialling.^[when?] Since the late 20th century, most international calls are dialed directly.

Calls are initiated by dialing the international call prefix for the originating country, followed by the country calling code for the destination country, and finally the national telephone number of the destination. For example, a landline subscriber in the UK wishing to call Australia would first dial the following sequence: 00 (the call prefix used in the UK to access the international service), followed by 61 (the country calling code for Australia), followed by the national subscriber number (without the trunk prefix 0 used for dialing within Australia).