As I was cataloging an item for the Andover Room, our local history collection, I noticed the cover listed Memorial Hall Library's phone number as simply "314," which looks more like a combination you'd see in Sudoku than a phone number!  While I do remember 7-digit dialing for local calls, 3-digit phone numbers are before my time.  Incidentally, that item had no date, but this type of phone number suggested to us that the item was from sometime around the 1940s or 1950s.  If that's piqued your interest like it did mine, you can learn more about the history of telephones with this article from Encylcopedia Brittanica or this video from Kanopy.