Patent US 5,832,057

Remote telephone coder 10R and local telephone coder 10L provide coding and decoding for voice communication between remote telephone set 11R and local telephone set 11L through telephone line 12. An eavesdropper listening in on a listening device such as remote extension telephone 14R or local extension telephone 14L hears normal "in-the-clear" conversation (uncoded transmissions) when both of the telephone coders are in the normal mode. However, when both of telephone coders 10R and 10L are in the code mode, the extension eavesdropper using an extension telephone or other unauthorized listening device 14 (shown in phantom) hears only coded transmissions. Further, a remote eavesdropper on remote extension telephone set (main portion 17M with hand portion 17H) employing remote extension coder 16R is also excluded from the coded transmissions between remote telephone coder 10R and local telephone coder 10L.