A minimum volume level can be set internally on the KB-212. See page 5, item 10 on the KB-212 user manual for a drawing of the internal adjustment control on the pcb. This allows the user to set a minimum listen level on the main volume control so that the user can't accidently turn off the listen level completely. To disable this option the user can adjust this control completely counterclockwise to disable this feature. Additional user manuals can be found on the Clear-Com web site.