An international dialling prefix is a telephone dialling code used to dial out of a country when making an international call. It's also called international access code, International Direct Dialling (IDD) code, exit code, international call prefix, etc. This is done to avoid confusing the country code with the first few digits of a local phone number.
These codes are different from the unique country calling code every country has which is used to dial into that country.
When making a call between countries, the caller must first dial the international dialling prefix, then the destination country calling code, then (usually) an area code, then the local subscriber's phone number.
The International Telecommunications Union (ITU) recommends 00 as the general standard for an international call prefix, and this has been implemented by a large number of countries, but not all of them International Telecommunication Union (ITU-T), E.164 - 12 (Page 22). This page only lists the exceptions, but rapid developments may make this list out of date. See External links for externally maintained lists which may be more up-to-date.
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 dialling prefix, to signify that the caller should use the prefix code appropriate for the country they are calling from International Telecommunication Union (ITU-T), E.123 - 7.1 (Page 9).
Many phones, particularly mobile 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 from any country they find themselves in.
The disadvantage of using the "+" sign is that there are some countries that have special international dialling prefixes that offer discounted overseas rates. The "+" sign will automatically be converted to the default dialling prefix which usually carries the regular overseas rates, hence you could forego the discounts.
If a caller in the USA or Canada wants to call the Mayor of Dunedin in New Zealand, they would dial 011 64 3 477 4000. (The generic international call prefix for North American Numbering Plan (NANPA) countries is 011, then the country code for New Zealand is 64, the area code for Dunedin is 3, and the local number for the Dunedin City Council is 477 4000)
If a caller in any of the countries of the European Union, China, India, South Africa (and many other Asian and African countries) uses the International Telecommunications Union (ITU) recommended 00 prefix, to call the same number they would instead dial 00 64 3 477 4000.
If a caller in Japan wants to call the same number, they would instead dial 010 64 3 477 4000.
If a caller in Australia wants to call the same number, they would instead dial 0011 64 3 477 4000.
Some telecommunication 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.
01x, where x is the international carrier selection code
Israel, where to encourage competition between international telephone carriers, a system of prefixes beginning with 01x was introduced, in addition to the 00 prefix. For example, if the subscriber dials 018-1-202-555-1212, the call will be routed to North America via Xfone.
012 – 012-Smile
013 – 013-NetVision
014 – Bezeq International
018 – Xfone
Singapore, this is in addition to the 00x prefix where its 01x counterpart allows cheaper rates
013 and 019 - SingTel
018 - StarHub
011 - Any country in the North American Numbering Plan (NANPA)
Finland, where to encourage competition between international telephone carriers, a system of prefixes beginning with 99 was introduced in addition to the 00 prefix. For example, if the subscriber dials 990-1-202-555-1212, the call will be routed to North America via TeliaSonera.
Phone cards and third-party services are used mostly for callers to enjoy effectively reduced rates. Each kind of service will have their own specific instructions on how to dial overseas phone numbers, which may or may not involve the usage of the usual dialling prefix for the country in which it is offered. These instructions are either printed at the back of the card or are posted on the service provider's website.