I was wondering how they managed such an impressive signal fan-out - I thought that telephones at that time were purely powered by acoustic energy moving the coil in the microphone. Turns out that the amplifying telephone repeater had just been invented a few years previously, enabling transcontinental phone calls and - in this case - splitting a signal to be played in many places.

It must have been an incredibly exciting time to be alive.

https://ethw.org/First_Telephone_Repeater