How have ferrites been used in telephone channel filters?

Ceramic ferrites were historically employed as channel filters in telephony. Their utility in this application stems from their high electrical resistance, which is achieved by oxidizing grain boundaries during processing. This property is critical because it reduces the occurrence of eddy currents—energy losses caused by rapidly fluctuating magnetic fields—allowing the materials to function effectively at higher frequencies.1

While ferrites were among the earliest materials used for this purpose, this specific application has largely been rendered obsolete by the development of solid-state filters and fibre optics.1