Hello,
 
have you heard of Minisip? It is a free softphone which concentrates on security. It is developed by the Royal Institute of Technology (KTH, Stockholm, Sweden).
I have tested to place encrypted calls with the commercial versions of X-Lite (called eyeBeam) and Zoiper (ZoiperBiz). While there were problems with eyeBeam I managed to place encrypted calls (signalling AND media) with ZoiperBiz via FreeSWITCH. Meanwhile there's an update available for eyeBeam which I have not tested. Maybe it works now, too.
.
> Do I understand this correctly that there aren't any softphone clients that support SRTP via SDES correctly ?  I found few clients that support SRTP
> here http://en.wikipedia.org/wiki/Comparison_of_VoIP_software but I haven't tested them yet.

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Alois Komenda
Fraunhofer-Einrichtung für Systeme der Kommunikationstechnik ESK