The volume controls only adjust what you hear on your own intercom station.
As an aid to giving instructions to people using the intercom, I used two differently coloured knobs for the volume controls. On each station I used a yellow knob for the intercom volume (yellow for yelling at each other), and a green knob for the audio monitor volume (such as might be used to listen to the program in the green room).
If you're making intercoms to be used in the dark, you could using two differently sized knobs, instead. e.g. Use a small knob on all intercom volumes, a bigger knob on the program monitor.
Whichever scheme you use, do it the same on all stations, so that one simple set of instructions explains all of the equipment.
To make things even more obvious, put the mike mute switch nearest the intercom volume control.
The mute switch controls your own mike. When switched on, it sends at a fixed level. Your volume controls only adjust what you get to hear.
Mikes on unattended stations should be switched off. Switch off your mike before taking your headset off, it's annoying to everyone else to hear someone crash their headsets on the table.
With some stations the mute control may be a toggle switch that stays where you set it. Others may used a spring-loaded switch that needs to be pressed to talk. Push-to-talk stations are only practical where operators have their hands free to press switches.