[russell_w_b]

Your wiring is all wrong. Looking at your mic, it looks like you're using the green wire as a common mic / rec wire, the white wire as the other rec wire and the red wire as the other mic wire. Leave this end as is.

At the telephone connector block, you need to connect your red wire (receiver) to T4, your green wire (common transmitter/receiver) to T5 and your white wire to T6.

Give that a go and see what happens.