Mitel Superconsole 9189-000-301-NA on the 200 product line the volume up and down does work for sure and from the docs:
"Adjusting the Handset and Headset Receiver Volume
The Volume keys adjust the volume of the console ringer and the volume
of the handset and headset receiver. Although you can have a handset
and a headset connected to the console at the same time, the receiver
volume for each is not individually adjustable; any volume adjustments you
make affect both the handset and the headset.
To adjust the handset and headset receiver volume while a call is in
progress and the console is not ringing, or while the console is idle:
• Press ∧ repeatedly to increase the volume or ∨ to decrease it".