Central unit shall include speaker restriction function which prevents interruptions. The number of conference units can be preset for authorized number of speakers at time. One, two or four persons can be allowed to talk at any one time to prevent confusion that could result from several participants all speaking at once. Speech system selection function sets speech when talk keys on conference units are activated exceeding set number of authorized speakers, microphone on/off operation can be set to any of following first-in-first-out, last-in-first-out or last-in-first-out after 2nd unit (microphone stays on for unit whose talk key was first pressed). Auto Mic-Off function prevents inadvertently leaving microphone on. With this function on, microphone will automatically switch itself off after 30-second period of silence after talk key was pressed.