PGPfone (Pretty Good Privacy Phone) is a software package that turns your desktop or notebook computer into a secure telephone. It uses speech compression and strong cryptography protocols to give you the ability to have a real-time secure telephone conversation. Secure voice calls are supported over the Internet, or through a direct modem-to-modem connection, or even over AppleTalk networks.

PGPfone 1.0 is freeware. It is available for both Macintosh (1.0b7) and Windows 95/NT (1.0b2).

http://www.pgpi.org/products/pgpfone/