Article ID: 2889647 - View products that this article applies to.
This issue occurs in the following scenario:
To resolve this issue, install the following cumulative update:
(http://support.microsoft.com/kb/2884632/ )Description of the cumulative update package for Lync 2010: October 2013
This issue occurs because Lync 2010 does not prompt the user to input credentials for the proxy server.
To work around this issue, you must use a proxy server that supports NTLM authentication.