The one I grew up with had a knob on each interior unit with a "door" position; you switched it to that, and used the PTT button to talk, then released it and it left the mic at the outdoor unit open. Nothing for them to push, and you could also use it to discreetly listen to any conversation going on outside.Venus Pax wrote:We have an intercom system, but it's old, and I'm not sure that a solicitor would know how to communicate using it. (You have to actually push a button and talk. Does yours work like this?)
I don't know the brand, but it was made in the late sixties or early seventies, and was entirely a wired, built-in system.