[tang-soo]

It was my understanding decades ago—that even the older landline phones could be switched on as a listening device if the authorities decided to do it. Even when it was on the cradle.

I remember an episode of 60-Minutes from the 1970s or 1980s that had a story about that very thing. They demonstrated how people with the right equipment could turn a phone that was hung up - ie in the cradle into a listening device. Even though the connection was "open circuit" there was enough induction that subtle vibrations were still available to be picked up. Never saw that technique ever mentioned again until today. :)

[WaltStuart]

Interesting. I didn’t see the 60 min thing. I just heard from the ET’s (Electronic Techs) in the Navy.

[bigbob]

That’s because it was BS. Depending a bit on the kind of landline phone, as after the Carterphone decision and phones that were not produced by Western Electric or another phone company manufacturing arm came on the market, they cut corners. But in the old dial and touchtone type single line phones in use in that timeframe, the hookswitch was a mechanical contact switch that absolutely disconnected both sides of the phone line when the receiver was in place. However many different kinds of listening devices could be embedded in the phone or wiring that indeed used the telephone microphone as a pick-up element, regardless of the phone status. Any kind of PBX or multi-line phones or more recent electronic phone systems have all kinds of vulnerabilities.