I occasionally worked in a place which had two buttons in the middle of the phone's handset. They had to be pressed to connect the microphone and speaker. My fingers would cramp if I had to be on the phone for more than a couple minutes.
I’ve used those buttons before. ...For crypto comm phones now retired from service.
STU-III
http://en.wikipedia.org/wiki/STU-III
An early telephone intercept program during the Cold War.
An excerpt from SpyCatcher.
"Taylor's laboratory was also busy working on a new modification to SF (Special Facilities), called CABMAN. It was designed to activate a telephone without even entering the premises by radiating the telephone with a powerful radio beam. It worked, but only over short distances.
They were also in the early stages of developing a device called a MOP. A MOP made a cable do two jobs at once - transmit captured sound and receive power. It was in its early stages, but it promised to revolutionize MI6 activity by removing the extra leads which were always likely to betray a covert microphoning operation. I spent a lot of time in my first years in MI5 ensuring the correct specifications for MOP, and it was eventually successfully manufactured at the MI6 factory at Boreham Wood. ..."