No U.S. area code starts with this

ONE

Think of the first digit you’ll never dial when making a domestic call.

U.S. area codes always start with a digit from 2 to 9—never 0 or 1—because the phone system reserves those for special purposes like operator assistance (0) or long-distance dialing (1). The clue plays on this rule by asking for the digit that doesn’t appear at the start, and "one" is the word form of that excluded number. It’s a straightforward trivia-based hint with no hidden wordplay, just a fact about how phone numbers are structured.