A trunk prefix is a number used to indicate the caller wants to dial between area codes (sometimes called a long distance call).
Some phone numbering systems work off a systems of area codes. Calls within the same area code can be made by dialing only the local phone number. Calls between area codes use a trunk prefix to show that the caller wants to call to a different area code.
In North America, the trunk prefix ‘1’ also doubles as the country code for countries within the North American Numbering Plan (Canada, the United States, most of the Caribbean, and American Territories in the Pacific)
In many other countries, ‘0’ functions as a trunk prefix. When calling many of these countries from outside the country, the leading trunk prefix ‘0’ is not included in the call.