Here is a the phone I'm talking about. While this looks almost exactly
like a Western Electric 500 set, and the dial and handset elements are
interchangeable with WE (and a number of other manufacturers) the internal
wiring is quite different.
Here is a picture of thephone with the cover off the handset splayed,
and the dial moved out of the way. Note that there are four wires running
from the handset: Two white which come from the earpiece, and a red/black
pair which come from the transmitter. They run across into a tangle of
wires at the network block.
Here's a detail of the wiring
block. The red and white wires are reasonably clear, but it's a bit tough
to see that the black one is plugged into to the third connector in the
back row (two to the left of the Green wire on the end).