For a while I have been struggling with Office Communicator 2005 and Windows Vista. Office Communicator 2007 is not supporting Office Live Communications 2005 server and there I am forced to use the Office Communicator 2005 client. All presence and chat features worked fine with 2005 but when it came for sharing your desktop or sharing a file the communicator started to complain about the firewall or me sitting behind a Network Address Translator (NAT).

Cannot establish the data sharing session with user1, either because user1 canceled the session, or because you or the person you are calling is behind a firewall or Network Address Translator (NAT).

 

How to resolve this issue?

Uninstall Microsoft Office Communicator 2005
Get the KB927853 hotfix from Microsoft. This update will install the NetMeeting 3.02 version which is compatible with Windows Vista.
Reboot the machine
Install Microsoft Office Communicator 2005

I scripted the Microsoft Office Communicator 2005 into a Altiris SVS application layer but after installing the hotfix it still didn’t solve my problem. Make sure you uninstall the Communicator first or delete the SVS layer and than reinstall the packages.