Show HN: I built an intercom for my 6 yo to keep us connected during quarantine
Back in the days of landlines, all corded phones in a house effectively acted as an intercom system.  Two or more users could pick up handsets, and their voices would be broadcast to one another.This worked even while on a call.  If you had siblings (or snooping parents), you learned to be alert for the click sound during a call which indicated that someone else had picked up a handset and was now eavesdropping.