A leading ‘1’ in a phone number is called a ‘trunk prefix’ – it is used to notify the circuits of the telephone network that the call is to be routed to a different region of the phone network.
It is only used within North American Numbering Plan (NANP) regions. Mongolian phone numbers use a leading ’01’ or ’02’ that functions as a trunk prefix, however this page does not include the Mongolian phone system.
It is similar in function to a leading ‘0’ that is used in heavily in Europe, Asia, Australia, and New Zealand, as well as some countries of South America and Africa.
On this page
- Where a leading ‘1’ is found
- Dialing within the same NANP country / region
- Dialing from outside an NANP country / region
- Mexican mobile phones
- Mongolian trunk prefixes
Where a leading ‘1’ is found
A leading ‘1’ is found in countries using the NANP – Canada, the United States, a majority of Caribbean countries & regions, as well as U.S. territories in the Pacific.
The Mexican phone system uses a leading ‘1’ for mobile phone numbers – when calling from another country it is included in the call sequence after the country code and before the local number.
The Mongolian phone system uses a leading ’01’ or ’02’ as a trunk prefix.
Dialing within the same NANP country / region
… long distance
When making a long distance call, the leading ‘1’ is included in the call sequence.
In many regions, phone networks have been upgraded so that if a call is placed without the leading ‘1’, the call will still be completed after a message informing the caller that the call will be charged at long distance rates.
When making a local phone call, the leading ‘1’ is dropped from the call sequence.
Dialing from outside an NANP country / region
… from another NANP country / region
When calling to an NANP country / region from another NANP country / region, the leading ‘1’ is included in the call sequence, however there is no ‘011’ that is common for placing an international call to a region outside of the NANP.
Example: The Metropolitan Museum of Modern Art in New York City, United States lists its phone numbers as (1) 212-535-7710.
The call sequence would be the same if the person calling the museum was located in Los Angeles, USA, Montreal, Canada, or Kingston Jamaica.
… from a region outside the NANP
When calling from outside the NANP, the leading ‘1’ is included however acts as the country code.
The next 3 digits (the first 3 digits of the 10 digit number) act as the area code that determine which country / region the call is routed to.
Mexican mobile phones
Mexican mobile phone numbers have a ‘1’ added to the call sequence when calling from outside Mexico.
When calling, place a ‘1’ between the Mexican country code (+52) and the mobile phone number that is being dialed.
Mongolian trunk prefix
Mongolia uses ’01’ or ’02’ as a trunk prefix.