Phil Zimmerman’s ZRTP (and similar protocols like MIKEY/SAKKE) manage to solve some usability problems for encrypted voice/video calls. The concept is simple: if a user reads a phrase on their screen that is the same as the phrase on your screen, you can be reasonably sure that the private keys that secure your transmission have not been intercepted with a man in the middle attack. Software like Jitsi and Redphone make good use of this and it is incredibly easy to use.