In those days the professors' offices were in clusters. The line buttons on the phone was for their own line, nearby lines, and a local INT with the secretary. If a phone kept ringing unanswered, either the dept secretary or another prof would answer it and take a message.
This was very common in offices everywhere back then.
But I suspect much of that has been eliminated through voice mail. If the prof isn't there, the voice mail takes the message automatically. There may be fewer secretaries (I suspect so, my employer has fewer). Also, email has eliminated the need for calls.