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ISD: International Subscriber Dialing

The full form of ISD is International Subscriber Dialing. This term describes an international phone call made by a person without depending on the telephone operator. It is also known as International Direct Dialing (IDD). International Subscriber Dialing (ISD) is assigned to all countries. It is a specific number. To make an international call, the subscriber must attach this code before the phone number of the specified person. When publishing phone numbers for international use, the international call prefix is omitted and the number must 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 need to be dialed in the country of origin. For example, ISDs of USA and India are +1 and +91 respectively.

Other full forms of ISD