A communication device is a piece of Guild technology that allows long-distance communication. It resembles a small glass square hanging from a pendant.
The use of communication devices gives the Guild a large tactical advantage over Anatoray and Disith, where voice communication between ships can only occur if they are connected by wires. Communication devices also allow the Guild to remotely issue orders for the Claudia Units of ships to be recalled.
A communication device is first seen when Lucciola gives one to Alvis before she, Claus, and Dio escape the Guild Fortress. Claus is able to use it to contact Lavie via the Silvana's Claudia Unit. The Claudia Unit capture operation carried out by the Alliance later in the series causes them to gain control of a large number of communication devices, allowing them to carry out the final battle against the Guild on more equal footing.