http://blogs.technet.com/mfugatt/archive/2008/06/09/description-of-the-update-for-the-unified-communications-client-api-sdk.aspxThe authentication protocol that is used by Microsoft Office Communications Server 2007 and its clients has been updated to require a signature in all messages. This includes the first message that carries the challenge response. The version number ofen-GBCommunityServer 2.1 SP1 (Build: 61025.2)