From the creator of PGP, Philip Zimmermann, now comes new encryption software specially designed to secure your VoIP connection: Zfone for Windows (Mac OS X, Linux).
The software is designed to virtually work with any SIP compatible softphone such as eyeBeam, X-Lite, Express Talk, GizmoProject, etc. All you need to do before establishing a secure connection is to make sure Zfone is running and the softphone is using SIP port number 5060.
Unfortunately the ZRTP protocol used by Zfone is not yet known by PBXs or softphones and so you cannot make a secure call to a land line. Your contact must also be running Zfone on his side, and so the two instances of Zfone will create an encrypted channel through which the VoIP data will flow secured.