A standard PSTN phone disconnects the microphone/speaker from the telephone circuit when on the hook. Originally, a switch pushed open by the handset in its cr

A standard PSTN phone disconnects the microphone/speaker from the telephone circuit when on the hook. Originally, a switch pushed open by the handset in its cradle. In practice it would not be possible with just about any phone. Might as well be unplugged. But I wouldn't say guaranteed for all models. Too many ways to design a phone. It's not too hard to design an electronic switch to disconnect from the circuit that would have some faint crosstalk imposing an AC signal on the line even when it's supposed to be an "open circuit". Probably easier than designing one that truly eliminates it, especially back in the day.