Every country has both a country calling code used to dial into a country, and an international call prefix or international access code used to dial out of the country. The ITU approved 00 as the general standard some time ago, and this has been implemented by a large number of countries but not all of them. This page lists the exceptions.
International phone numbers are usually written with a plus sign (+) before the phone number representing the international call prefix. The plus sign indicates the code required to dial out of your local area; for example, in the United Kingdom, you use 00 to dial out and 1 to call the United States with the complete format being 00-1-xxx-xxx-xxxx. Some phones allow this to be entered automatically, often by holding the '0' key (most GSM mobile phones) or with two consecutive presses of the * key, which avoids having to know the international call prefix.
For example a UK phone number written as +44 1234 567890 would be dialled as 567890 from the '01234' linked numbering scheme exchanges in the UK; as 01234 567890 from elsewhere in the UK; and as 00 44 1234 567890 from countries using the 00 international access code. For countries using a prefix other than 00, simply substitute that for 00 in the last example; for example, from North American Numbering Plan countries (including the United States and Canada), dial 011 44 1234 567890.
