Overview#International Direct Dialling (IDD) is a Telecommunications dialling code used to dial out of a country when making an international call.
International Direct Dialling also called international access code, International Dialling prefix 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.
When making a call between countries, the caller must first dial the International Direct Dialling, then the destination Country Calling Code, then (usually) a National Destination Code (Area Code), then the local Subscriber Number.
The International Telecommunication Union (ITU) recommends 00 as the general standard for an International Direct Dialling, and this has been implemented by a large number of countries, but not all of them International Telecommunication Union (ITU-T), E.164.
Plus (+) sign#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 Direct Dialling, to signify that the caller should use the International Direct Dialling 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 Direct Dialling, 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.