NSA's STU-III, KY-57 and SCIP are examples of systems that operate over existing voice circuits.
Voice of America | The Village Voice | Voice acting in Japan | voice-over | voice acting | Secure Shell | voice | The Voice | His Master's Voice | Voice acting | Voice of Russia | Secure Hash Algorithm | The Voice UK | The Voice of Holland | Secure Real-time Transport Protocol | Voice over IP | Secure Digital | The Voice (U.S. TV series) | Secure Computing | Interactive voice response | George Ball (voice actor) | European Voice | You're the Voice | Voice-over | Voice for Life | The Witches' Voice | The Voice (TV series) | The Voice of Ireland | The Voice Israel | Mark Elliot (voice-over artist) |
SIGSALY, secure voice transmission system; sometimes called "X System"