It is just a basic amplified speaker that will deliver whatever you send to it via the tip/ring pair to allow others in the room to hear both sides of the conversation. They can't talk back, though. They offered many purposes, but the most common was to just connect the tip/ring to the handset receiver terminals inside of a phone.

The black/yellow pair requires 13.6VAC to power the amplifier.


Ed Vaughn, MBSWWYPBX