How does a telephone hybrid transformer work?

Question

Suppose the following circuit:

I can see how an incoming signal (Kim's voice) wouldn't be coupled with the lower pair, but I can't see how an outgoing signal (Ron's voice) only goes through the lower pair.

When the signal from the upper pair reaches the hybrid then one part of it goes to the 2 wire telephone line (seeing a line impedance of Z1) and the other part (right inner inductors) does the same except it will 180° out of phase when it reaches the lower pair (because the impedance Z0 perfectly matches Z1). As such Kim's voice won't travel back.

On the other hand, I don't understand how a signal coming from the left phone would only travel through the lower pair.

Answer 1

On the other hand, I don't understand how a signal coming from the left phone would only travel through the lower pair.

I've concluded that some of the signal from the telephone (Ron's voice) does reach the transmit amplifier (the source of Kim's voice) BUT this is just not that a big deal because, the important thing is that Kim's voice (port 2) does not get transmitted to port 3 (The receive path for Ron's voice).

Here is a better diagram showing the impedances not shown above: -

Also here is what wiki says about hybrid transformers: -

Double transformer hybrid

When both the 2-wire and the 4-wire circuits must be balanced, double transformer hybrids are used, as shown at right. Signal into port W splits between X and Z, but due to reversed connection to the windings, cancel at port Y. Signal into port X goes to W and Y. But due to reversed connection to ports W and Y, Z gets no signal. Thus the pairs, W & Y, X & Z, are conjugates.

Note 1 - Port Y is the balancing impedance shown as Z\$_B\$ in the top diagram in my answer.

Note 2 - Port W is the telephone line

Note 3 - Ports X or Z are interchangeable as the transmit output or the receiver input.