Historically Significant Products
Scramblers were used to secure voice traffic during World War II, but were often intercepted and decoded due to scrambling's inherent insecurity. The first true secure telephone was SIGSALY, a massive device that weighed over 50 tons. NSA, formed after World War II, developed a series of secure telephones, including the STU I, STU II and STU-III, as well as voice encryption devices for military telephones.
Other products of historical significance are PGPfone and Nautilus (designed as a non-backdoored alternative to Clipper, now officially discontinued, but still available on SourceForge), SpeakFreely, and the security VoIP protocol wrapper Zfone developed by the creator of PGP.
Scrambling, generally using a form of voice inversion, was available from electronic hobbyist kit suppliers and is common on FRS radios. Analog scrambling is still used, as some telecommunications circuits, such as HF links and telephone lines in the developing world, are of very low quality.
Read more about this topic: Secure Telephone
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