Just as an addendum, the letters predate touch tone phones by a lot. They were originally used for the central office prefix, which in a lot of smaller places was also just the town name. If you were within the town you could just use the 4- (or later 5-) digit phone number of the person you were calling, but if you wanted to call the next town over you would need to dial the 2 numbers corresponding to the letters or tell the operator the name and number, like “Lakewood 2697”. That’s my understanding, anyway, from talking to people who lived in that time or seeing it in movies.