YT-468 interphone is very popular use. It can't use alone. It's always used as a set including this intercom, a signal Transmitter, one earpiece and a signal receiver. With a signal Transmitter, one person speak with the interphone and another with the signal receiver and earpiece can hear the voice in another room or some places 50m away.
Usually this set of interphone is used with IR marked cards and IR camera. The guy who see the marked cards with a screen in another room can tell his partner who is playing in the poker room information that he get from the invisible ink marks.