International direct dialling

International direct dialing (IDD) or international subscriber dialling (ISD) is placing an international telephone call that is dialed directly by a telephone subscriber, rather than by an telephone operator. The term international subscriber dialling was used in the United Kingdom and Australia until the terminology was changed to international direct dialling. 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.

When phone numbers are published for international use, the international call prefix is omitted, and the number should start with a plus sign (+) followed by the country calling code. The plus sign indicates that the country code follows and that an access code may have to be dialed in the originating country.

The first transatlantic direct dial telephone call was made by Sally Reed in Dedham, Massachusetts to her penpal, Ann Morsley, in Dedham, Essex, in 1957. It was witnessed by Reed's teacher, Grace Hine, Dedham's former chief telephone operator, Margaret Dooley, and several representatives of New England Telephone and Telegraph Company.

A fictitious number in Sydney, Australia is (02) 3456 7890. It is published in the form +61 2 3456 7890 for international use. In the countries participating in the North American Numbering Plan (NANP), such as the United States, Canada, and some Caribbean nations, this number is dialed as 011 61 2 3456 7890, with 011 being the international call prefix used in the NANP and 61 being the country calling code of Australia. From most of the rest of the world, the international access code is 00, so the call is dialed as 00 61 2 3456 7890, as recommended by the regulations of the ITU.

...
Wikipedia