International call prefix

An international call prefix or dial out code is a trunk prefix used to select an international telephone circuit for placing an international call. It is often referred to as IDD Prefix (International Direct Dialing)—a country will typically have a NDD Prefix as well (National Direct Dialing). The international dialing prefix must be dialed before the country calling code and the destination telephone number. It is synonymous with international access code or exit code. The international call prefix is part of the telephone numbering plan of a country for calls to another country.

The International Telecommunication Union (ITU) recommends the sequence 00 as a standard for an international call prefix, and this has been implemented by many countries, but not all of them. Some countries use the 00 prefix which is followed by the international carrier code.

When phone numbers are published for use abroad they typically include the country calling code, but show a plus sign (+) prefix in place of any international call prefix, to signify that the caller should use the prefix code appropriate for their country.

Many phones allow this "+" to be entered in their saved number lists, often by holding down the '0' key (most GSM mobile phones) or with two consecutive presses of the * key. When making a call the system then automatically converts the "+" to the correct international prefix, depending on where the phone is being used, which enables callers to use the same stored number when calling from either their own country or any other.

Calling the Dunedin City Council in New Zealand:

Some telecommunications service providers or mobile network operators (wireless carriers) have different international call prefix codes which callers or subscribers can use to route calls through specific networks, sometimes with special pricing arrangements.

Many countries also provide alternative dialling arrangements for calls to neighbouring countries.

...
Wikipedia