We are using this with a 2830 phone set. The current wiring is without the connecting block. We are connecting the 108, 680 and 85B1 directly to the M16 cable. This configuration was used for decades.
This system is wired for the purpose of a Live TV Talk show. We are taking audio from the speaker set and feeding that to a Mixer in the TV Master Control room.
I swapped a known good speaker from another studio, and tightened the power supply terminals. I measured the voltage to be about 22.5Vac at the terminals and at the connector pins 10 and 36.
Now, I'm able to hear the caller at the speaker, but the caller cannot hear me. Also, the 680 connects/disconnects calls and controls volume of the speaker.