Technically it's the country code for the North American Numbering Plan, which is used by several other countries as well as the US.
But in this context it'd be the first digit of the area code, with no country code being used because the call is within the US. There are no area codes in the the North American Numbering Plan that start with a 1.
"The syntax rules for area codes do not permit the digits 0 and 1 in the leading position."
My guess would be it's to avoid ambiguity with the fact that 1 is also the country code. If I recall correctly, historically, dialing the 1 was necessary for any long distance call (even if not international).
>If I recall correctly, historically, dialing the 1 was necessary for any long distance call (even if not international).
You recall correctly. I haven't had a landline for a number of years now but I think it was still required latterly when I still had one. Don't think it was ever needed on cell (or maybe even valid) when I first got one at some point in the 1990s.
It used to be the case that the middle digit of an area code had to be a 0 or a 1. All the O.G. "cool" area codes like 212 are in this format, and the less desirable new area codes like 646 are not (yes, this is an accidental Seinfeld reference).
I think that’s at least in part because some 555 prefix numbers have been assigned for non-directory information uses (I have a vague notion of seeing this for some toll-free numbers).
