Running stuff at 12/24 volts may need another set of house wiring installed (higher current ratings for everything).
The most common source is 50 or 60 Hz hum from power transmission systems (house wiring, etc.).
Messages for the intelligent plug are carried by signals on the house wiring.
Their sales point is that the equipment can plug straight in without altering the house wiring.
It does not seem like too far a stretch to send that signal over house wiring to the chips contained in the various appliances.
It only takes two wires to connect a phone, but most house wiring contains four wires.
"But I'm not using the house wiring so I still need daylight."
It uses your house wiring to carry the audio.
These devices are only designed to safeguard the house wiring in the event of a short circuit.
Power line communication units that send signal over house wiring have been referred to as "wireless" intercoms.