The Telephone Cable is a historical item which can be found by Emile in Vauquois' mines during Chapter 3.
Location[]
During the dynamite and basket puzzle, light a stick of dynamite in the basket and quickly move it all the way right - the explosion will dislodge some loose dirt and this item, which you may need to then collect from the dog.
Description[]
"Telephone cables were originally made with a single wire, but this posed serious security issues. Special divisions were created to listen into conversations. All eavesdroppers had to do was hook up to the wire and listen in, undetected. To remedy the problem, telecom engineers created a closed system with a second wire, which made eavesdropping much more complicated."