Each of our staterooms, the crew quarters and the flybridge have an intercom that communicates with the master unit at the pilot house helm. So, it really only works to talk to someone at that master station, but the slave units can initiate the communication. We use it mostly at night, when we are always running from the pilot house anyway, to give shift change reminders, but I also use it to talk with the pilot house helm from my stateroom, where I have a redundant radar and chart display. Theoretically, whoever is at the help could call me if need be, but that never seems to happen.
On my last boat, we had telephone type intercoms (any station could call any station), but we never used it.