ISD Full Form is International Subscriber Dialing. ISD is a term that describes International calls dialed directly by the dialer. In other terms, there is no operator involved in aiding the call though it is International. ISD is also called as IDD or International Direct Dialing, as it is calls made directly by the subscriber without the assistance of an operator.
In order to dial ISD calls, one has to prefix the country code followed by the area code, which in turn is followed by the number of the receiving personal abroad. Almost all International calls require ‘00’ or ‘+’ as the first character of the dialed number. Currently, almost every service provider offers ISD facility in both mobiles as well as landlines. However, the charges may differ.