A trunk prefix is a digit sequence to be dialled 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.
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Making a domestic (national) telephone call usually requires the dialling of a single or two-digit national trunk prefix preceding any area codes and the destination subscriber number. In most countries, the trunk prefix is 0.
For international telephone calls, the national trunk prefix is not dialled; instead, an international trunk prefix (or "+") is typically required, followed by the destination's country code.
This example assumes that a call is to be made to a customer in the Australian state of Queensland with the local number 3333 3333 and the area code 7.
A caller from outside Australia must dial the international call prefix of the originating country and the country code (61 for Australia), then the area code (7), and then the local subscriber number. Therefore, a caller in the UK must dial 00 61 7 3333 3333, while a caller in the US must dial 011 61 7 3333 3333.
Calling inter-area (within Australia) (e.g. from Western Australia with area code 8), a caller need not dial an international trunk prefix or a country code. Instead, 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 can recognise the area of origin of the call, its direction and destination to facilitate communication faster. If the full international number is used, then this full international number may be dialled from nearly any telephone within a recognisable 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, a telephone number is represented as +61 7 3333 3333, and includes only the digits that must be dialled internationally. The plus sign indicates that an international trunk prefix is first dialled and, therefore, a country code follows.
