There's a book "Everything Happened Around The Switchboard" that may still be available from Mike Sandman and tells of that kind of operation.
Every manual switchboard I've seen, from PBX to central office, has an audible alarm that the operator can turn on or off. When turned on it sounds when there is an incoming call that needs attention. It also lights a signal lamp on the board. This circuit is always called a "night alarm".
jhhaynes at earthlink dot net
[TELECOM Digest Editor's Note: Yes, and a foot pedal/switch allowed the operator to temporarily mute the audible alarm also. PAT]