The Full form of ISD is International Subscriber Dialing. ISD is also known as International Direct Dialing (IDD). This term describes an International telephone call dialled directly by a person (or a telephone subscriber) rather than relying on the telephone operator. Every country is assigned an ISD number. It is a code or a specific number.
A telephone subscriber has to add this specific number/code before the individual’s phone number to make an international call. The term ISD (International Subscriber Dialling) was used in Australia and United Kingdom until the terminology was changed to IDD (International Direct Dialling). Most of the international calls are dialed directly since the late 20th century. The international call prefix is omitted when telephone numbers are published for international use 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. A telephone number in Sydney, Australia is (02) 2453 6789. It is published in the form +61 2 2453 6789 for international use. The international access code is 00 from most of the rest of the world, so the call is dialed as 00 61 2 2453 6789, as recommended by the regulations of the ITU.