Because Office Communications Server 2007 R2 uses mutual TLS (MTLS) authentication, each Group Chat Server (running the Lookup service, the Channel service, and the Web service) requires a certificate issued by the same certification authority (CA) as the one used by Office Communications Server 2007 R2. The certificate is initially configured during installation, but you can change it by using the Group Chat Server Configuration Tool.
To change the certificate used to communicate with Office Communications Server
Click Start, point to All Programs, point to Administrative Tools, and then click Office Communications Server 2007 R2 Group Chat Server.
In the Group Chat Server Configuration Tool, on the Filemenu, click Configure Server Settings.
On the left navigation pane, click Machine Wide.
Click Browse, and then select the certificate to be used.