The communications are relayed by speakers spread through the laboratories. The speakers always come in two, and they are sometimes accompanied by a screen (although they could also consist of a speaker and a microphone, possibly to be activated by the small button always present under it). These screens are never used by GLaDOS during the game, hinting they were originally used by the Aperture Science scientists. Instead, they are completely black, with an Aperture Laboratories logo constantly fading from orange to yellow on the bottom left, acting like some sort of screensaver (the button present under the speakers follows the same color fade).
Throughout the Test Chambers, the communications are apparently made in a different way, as no speakers can be found. Security cameras also complete the surveillance.
The devices are often found near doors, for convenience or possibly because they have other abilities, such as locking doors.