"Don't Jiggle the Switch Hook"
While doing some research for a project I'm working on, I came across this video in the Prelinger Archives: a 1927 film providing instructions on how to use the new dial telephones.
My favorite line:
"Remove the receiver carefully so as not to jiggle the switch hook."
The original piece can be seen in context at http://www.archive.org/details/HowtoUse1927