This social signal predates cell phones. There was a bit about it in an episode of Seinfeld, “The Maid” (1998)
> The episode also featured the New York area code 646. When the 212 area code ran out of numbers, 646 was created. Elaine gets a new number with the 646 area code. She is not happy with the new number because she believes the area code makes it too long to dial. She is proved correct when attempting to give her number to a man in the park. Initially eager, he reads the number, asks if it is in New Jersey. Her response is, "No, it's just like 212 except they multiplied every number by 3…and added 1 to the middle number." He says he is already in a relationship and walks off.