didn't someone tell me that telco equipment had 40vdc racks once?

[Jonathan Hutchins]

During the 60's & 70's, black and yellow were used for in-home wall warts
that provided power to things like Princess Phones with light-up dials. 
Eventually a bad batch of these caused a number of house fires, and the
system was discontinued, made unnecessary as solid state technology needed
less current and could take power off the line voltage.