Hotline automatic ring down circuit complete with 90 volt 10-30 cycle ringer. Can work with any standard dial system telephones,

I have been looking for the old Ramsey ring down circuit, or similar private branch exchange ring down hot line circuits. These work with any old telephones, hooked to a private exchange that is NOT connected to the outside line. When the receiver is lifted from one telephone, all the other telephones ring, whether it is 1 other phone or 100 telephones. These were popular as hotlines to the desk in hotels, and intercoms in businesses, factories, warehouses, etc. Most schematics I see do not have the 90 volt ring supply, or any relays to activate, and deactivate it. These were quite common many years ago. I find it very hard to believe a good schematic does not exist. These were also called "hot lines". I always called them a "PBX ring down".