Jon Blund, the baby call that rings your phone

Field notes

The setup assigns one phone to the crib and uses its microphone to detect sustained crying, while a second phone receives a conventional mobile-network call. The listener must stay plugged in with its screen on, and the stated range depends on having mobile signal rather than maintaining a dedicated radio link. The page also says it filters sounds such as fans, traffic, and doors closing, though those detection claims are presented by the site rather than independently verified.