Old Nutone Intercom

I have an old Nutone Intercom system in a house that we buying. There are units all over the house but I can not find a "Master" unit. It is a three wire system and the intercoms have a volume control, a talk button for inside speakers, and talk/listen buttons for the door speaker. They are ivory colored with wood grain accents. They are not working even though all are connected. Do all systems require a master? What are my options? Thanks Rich

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