International direct dialing (IDD) or international subscriber dialling (ISD) is placing an international telephone call, dialed directly by a telephone subscriber, rather than by a telephone operator. Subscriber dialing of international calls typically requires an international call prefix (international dial-out code, international direct dial code, IDD code) to be dialed before the country code.

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. For example, a landline subscriber in the UK wishing to call Australia would first dial 00 (the call prefix used in the UK to access the international service), then 61 (the calling code for Australia), then the Australian number (omitting the leading zero).

When telephone phone numbers are published for international use, the international access code is omitted, and the number is listed to 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. In March 1970, the United States introduced a new nationwide system, called International Direct Distance Dialing (IDDD), as an extension of Direct Distance Dialing (DDD) that was inaugurated in 1951 in Englewood, New Jersey.