Dial a leading ‘1’ when sending a fax only if you would dial a ‘1’ for a regular phone call.
As fax works over the phone network, all fax numbers are simply phone numbers and should be dialed the same way as a regular phone call.
When calling locally within the United States and Canada: There is no need to dial a leading ‘1’. Local calls can be 7 or 10 digits depending on whether the region has 10 digit dialing.
When calling long distance within the United States and Canada: The leading ‘1’ is added. Long distance calls have a leading ‘1’ followed by 10 digits (the first 3 digits are the area code).
When calling to the United States and Canada from outside North America: Include the leading ‘1’. To call to the U.S. or Canada you will need to dial your country’s exit code (00 for many European countries, 0011 for Australia), followed by ‘1’ (the country code for the U.S. and Canada), followed by the 3 digit area code and the 7 digit local number.