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

[James Sissel]

    The term "Ring" refers  to the ring of a plug that is the direct ancestor of what we use for musical instruments today

So "Tip" refers to the TIP of that same plug. 

The other odd details of note are  that in the original "Bell System" most phone wiring for POTS was on either Red Green Yellow 3w cable or Red Green Yellow Black 4c wire.

The oldest connections used Red and Green for the talk and dial circuit and the Yellow was often used for either Ringing or party line user identification -where the GREEN wire was POSITIVE and called TIP. 
The RED wire was Negative and called Ring
The Yellow wire was called SLEEVE or the rearmost part of the plug  with BLACK as a second sleeve in my recall of the systems. Curiously when I fact checked myself the Yellow wire use seemed to be regional and not totally consistent even in the same region.. 

  More worthless information.  I worked for the teleco in Muscatine, IA for a summer in 1977.  I got to remove people's phones and install the modular plugs.  They only used the red and green.  The yellow and black were used for the power for the lights in the princess phones.  They had lighted dials so you could see them at night.  They were wired to a small transformer usually in the basement.  So if the power went out the phones would still work but the light in the princess phones would go out.
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