The telephones on the desk

To bug le the phone

The switch board could probably listen in with old phones. But the mouthpiece might be just a horn to channel sound, the mic is inside.

From IMDB trivia:

The telephone mouthpieces in the newsroom are missing because there was a general belief that these candlestick phone mouthpieces were a source of tuberculosis germs. Bensinger is the only character who has his own mouthpiece that he guards from everyone else using. The glass mouthpiece he uses was considered to be less prone to germs.