Zfone is software for secure voice communication over the Internet (VoIP), using the ZRTP protocol.
It runs in the Internet Protocol stack on any Windows XP, Mac OS X, or Linux PC, and intercepts and filters all the VoIP packets as they go in and out of the machine, and secures the call on the fly.
The Zfone software detects when the call starts, and initiates a cryptographic key agreement between the two parties, and then proceeds to encrypt and decrypt the voice packets on the fly.
Zfone describes itself to end-users as a "bump on the wire" between the VoIP client and the Internet, which acts upon the protocol stack.
It appears that Zfone development has stagnated, however, as the most recent version was released on 22 Mar 2009.