A trunk prefix is the number to be dialed in a domestic telephone call, preceding any necessary area codes and subscriber numbers. When the number is called from overseas, the trunk prefix is omitted by the caller. In most countries, such as Australia, Germany and the United Kingdom, the trunk prefix is 0. In the North American Numbering Plan it is 1.
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Assume a call is to be made to someone in the Australian state of Queensland.
A caller from outside Australia would dial the international access number (international call prefix) of the originating country (00 for many countries, 011 from NANP areas), then dial the country code (in this case 61), omit the trunk prefix, then dial the area code (in this case 7), and then the local number. So a caller in the UK would dial 00 61 7 3333 3333, while a caller in the USA would dial 011 61 7 3333 3333.
Calling interstate (e.g. from Western Australia), a caller would not dial an international access number or country code, but dial the trunk prefix (in this case 0) then the area code, followed by the telephone number: 0 7 3333 3333
Calling from within Queensland, one simply dials the telephone number: 3333 3333.
When conducting business (e.g., on business cards or stationery, etc.), that number should be written as +61 7 3333 3333 and should include only the digits that must be dialed from abroad. The + sign is used in place of the international access code. It has become common practice to write the number with the trunk prefix in parentheses, for example: +61 (0) 7 3333 3333. However, someone calling this number from the USA would probably dial all the digits, resulting in a failed call. ITU-T Recommendation E.123 states that parentheses should not be used in the international notation.