I saw a photo the other day that was dated 1973, and the phone number on a billboard still had a 2-letter prefix then 5 numbers. The billboard was from New York state as I recall. I grew up in Cincinnati, and don't recall ever seeing/hearing/learning any phone numbers that weren't just 7 digits long, all numerals. 1. when did US phone numbers become 7 digits? 2. when did the 2 letter prefix go away? 3. what was the purpose of the 2 letter prefix?