ISD is the short form for International Subscriber Dialing. Its other name is International Direct Dialing. It refers to a call made internationally without engaging the operator.
Every country has its own ISD assigned to it. For an international call to go through, this number or code has to be added before the recipient’s phone number.
The caller dials the international call prefix of the country of origin, followed by the calling code of the destination country and then the national telephone number of the recipient.
However, the ISD is omitted from phone numbers when they are published for international use. Instead, a plus sign (+), which replaces the ISD, is placed before the country calling code. The plus sign is used to show that what follows is the destination country code, and that the call may need to involve an access code in the country of origin.
That is exactly how it works.