telephone and telegraph system linked the various parts of the fortress. krepost sveaborg had both a regular telephone system and a second, independent telephone network dedicated for fire control. the primary telephone exchange was located on iso mustasaari. the various island forts had their own telephone exchanges, while kuivasaari and isosaari had central coastal telephone exchanges with links to other main line of defence forts. the telephone cabels on main line of defence were usually buried, while second line used overhead lines. besides telephone optical telegraphs, signal lamps and signal flags were also used.