[VoIP] 4-Wire Telephones?

[Rusty Dekema]

Good evening,

Recent discussions on the CNET list have reminded me of the existence
of 4-wire telephone sets. I also recall that my grandmother's house
was originally wired with 4-wire telephone jacks. (It was converted to
a 2-wire system longer ago than I can remember [having been born only
in 1982], but when they did so, they left one of the old 4-wire
connectors in place in the upstairs bedroom, and I have also observed
that, despite the fact that the house was built well before World War
II, it is wired with 4-wire telephone cabling.)

This makes me wonder, how did 4-wire telephones work? I would guess
that two wires were used for Tx and TxGND, and the other two for Rx
and RxGND, but beyond this? Were all four wires switched to the called
party's equipment at the CO and over the long distance network? Or was
it converted to a two-wire call at the CO? If so, was this done only
for long distance calls, or for local calls as well?

I know less about the "old school" telephone network than I would like
to, so I am always up to learn about it. If you have any web or print
references for me, I'd appreciate those as well.

Thanks,
Rusty