"Like the telephone, the punchy “hello” was a liberator and a social leveler. “The phone overnight cut right through the 19th-century etiquette that you don’t speak to anyone unless you’ve been introduced,” Mr. Koenigsberg said. And “hello” was the edge of the blade. “If you think about it,” he said, “why didn’t Stanley say hello to Livingston? The word didn’t exist.” Neither did the simple and elegant “dude,” so Stanley was thrown back on the formal “Dr. Livingston, I presume.”"
— NYT: Great ‘Hello’ Mystery is Solved
Apparently today I am compulsively researching the minutiae of telephone history? (For example, did you know that 212 is NYC’s area code because it’s the easiest combo to dial on a rotary phone? Followed by - yep, you guessed it - 213 for LA and 312 for Chicago? THE MOAR YOU KNOW.)