In Exploding The Phone I wrote about pioneer phone phreak David Condon who modified a Davy Crockett Cat and Canary Bird Call Flute to generate the 1,000 Hz tone -- modulated by a 20 Hz warble -- that was used as the "ring forward" signal on certain long-distance circuits back in the 1950s.
Turns out: the Bell System had their own version of this whisle! Made of metal, not plastic:
In the very best creative naming traditions of the Bell System it was called the "1A Whistle" and it generated the same "thousand by twenty" ring forward tone. It was used by technicians in the field to send a ringing signal to terminating offices.
More on the 1000/20 signal can be found here, including audio of what it sounded like. (On the linked page, scroll down to the "Ringer-Oscillator" photo and click on the image to listen to it.)
Thanks to the Yahoo "coldwarcomms" group member Ozob for bringing this to my attention and to Paul Wills of the telephonecollectors.org web site for posting the photo and audio files.