Passing security credentials to the third server

I have the following scenario.  An user uses the desktop application to call our WCF Service which has windows authentication.  The WCF Service calls the Office Communication Server (OCS) in order to do some custom work.  
When the WCF Service calls OCS we have to pass an instance of NetWorkCredential into the call.  I want to pass in the original calling users NetworkCredential object into the OCS call rather than constructing it using a custom username and password.  How do I get access to the calling users NetworkCredential object from the WCF service and pass it to the OCS call?


Thanks
Arjuna.
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