***** Moderator's Note *****
> Perhaps you can confirm something: an old AT&T tech told me that
> before N carrier was introduced, toll cables were so precious that it
> was common to use duplex/simplex repeating coils to make Ghost
> circuits from phantom circuits, and to obtain "Wraith" circuits from
> Ghosts. He told me the Wraiths carried telegraph traffic, sometimes
> working against ground and sometimes via other Wraith circuits, and
> that testmen had to know American Morse code because the testboards
> used Morse sounders and keys for communications between offices. True?
I don't know all the technical details but basically the above sounds correct. Before carrier toll circuits were rare.
There are two excellent books I strongly recommend which go into detail about this stuff (and where my questions arise from): They are:
"A History of engineering and science in the Bell System" / prepared by members of the technical staff, Bell Telephone Laboratories
vol 1. The early years (1875-1925) vol 3. Switching technology (1925-1975)
Vol 1 covers the early toll circuits, instruments, and switching up to panel and would answer the questions above. Vol 3 covers later switching--#1, #4, #5 crossbar and ESS.
(Vol 2 covers military products).
I also recommend two books on IBM technical history: "IBM's Early Computers" by Bashe et al and "IBM's System/360 and System/370" by Pugh et al.
I will note all the books are technical/research histories and tend not to cover manufacturing, marketing, or financial issues.