A trunk prefix is a digit sequence to be dialed before a telephone number to initiate a telephone call for the purpose of selecting an appropriate telecommunications circuit by which the call is to be routed.
Making a domestic (national) telephone call usually requires the dialing of a single or two-digit national trunk prefix preceding any area codes and the destination subscriber number. In most countries, such as Australia, Germany, Japan, the Netherlands, Sweden, and the United Kingdom, the trunk prefix is 0. In the North American Numbering Plan (NANP) states, such as the U.S. and Canada, it is 1. For international telephone calls, the national trunk prefix is not dialed; instead, an international trunk prefix is typically required. Many countries use the sequence 00, but in the NANP it is 011.
A caller from outside Australia must dial the international call prefix of the originating country, plus the country calling code (61 in the case of Australia), plus the area code (7 in this case), and then the local subscriber number. Therefore, a caller in the U.K. must dial 00 61 7 3333 3333, while a caller in the U.S.A. must dial 011 61 7 3333 3333.
Calling inter-area (within Australia) (e.g. from Western Australia--area code 8), a caller need not dial an international trunk prefix or a country code. However, the caller must at least dial the Australian trunk prefix (0) followed by the area code (7) and then the local subscriber number: 07 3333 3333. Calling from within the Queensland (7) area, a caller need only dial the telephone number: 3333 3333.
However, since the early 21st century, the majority of telephone systems worldwide are able to recognize the area of origin of the call, its direction and destination in order to facilitate communication faster. If the full international number is used, then this full international number may be dialed from nearly any telephone within a recognizable landline or mobile network. This has become particularly vital for users of mobile phones.
When conducting business, e.g., for display on business cards or stationery, the number would be +61 7 3333 3333 instead, and include only the digits that must be dialed from internationally. The plus sign is used to indicate that an international trunk prefix is first dialed and, therefore, a country code then follows. It has become common (but incorrect) practice to write the number with the national trunk prefix in parentheses, for example: +61 (0) 7 3333 3333. However, someone calling this number from the U.S. may mistake the national trunk code for a single-digit area code, as NANP area codes are often written in parentheses, dial all the digits and result in a misdirected or failed call. ITU-T Recommendation E.123 states that parentheses should not be used in the international notation to avoid such confusion.
The number next to the country in the list below denotes the country's national trunk prefix.