I assume there would have been a microphone to place next to the telephone bells, which in those days were located in ringer boxes mounted on the wall or under the desk. When the mic picked up a loud noise it would operate the hookswitch release and begin the process of playing the greeting and taking the message.

Since the device would have been purely acoustically coupled, there would be no electrical connection to the telephone line, and thus no violation of FCC regulations (though telcos did attempt to regulate acoustically coupled devices, leading to the Carterfone case that broke the barriers entirely and set the stage for deregulation to this day.

One little problem however. In the event of a natural disaster (storm, earthquake, etc.), there would be sufficient noise in the vicinity of the ringer pickup mic as to cause it to trip the hookswitch, thereby signaling the operator at the telco central office. She (it was always She in those days: female voices are more intelligible over telephones than male voices, due to the harmonic structures of the voice) would plug in and, upon getting no reply, assume something bad had happened unless the machine started playing its greeting. Operators’ time could thus be wasted in an emergency, an undesirable “unintended consequence.”

BTW, the phone shown in the picture has about the same sound quality as a modern cellphone, or perhaps slightly worse. The model that came after it, with the one-piece handset, has better sound quality than a modern cellphone. Speaking of “nothing new under the sun.”

Set in the late 1930s, the movie shows his “Rube Goldberg-like” answering machine using disks.

(I’m dissappointed the IMDB.com website doesn’t list the text of his outgoing message. It’s something to the effect of “”Your message is being recorded. Speak now.”)

PS:Wikipedia credits Doc Savage (created in 1933) with inventing the answering machine.

PSPS: I seem to recall a late 1950s-early 1960s electronics magazine describe a ‘build-it-yourself” answering machine using small reel to reel tape recorders. Similar to the Doc Savage design.

PSPSPS: Same magazine… (maybe even the same issue?) A ‘build-it-yourself” magneticly levitated globe. (OK… a 2 inch globe from a pencil sharpener, but still.)