Consider the following scenario:
You sign in to Microsoft Lync 2010 in an organization that has a proxy server deployed.
The proxy server requires all outgoing web access requests to be authenticated by using basic authentication.
A Lync 2010 user in a non-federated organization sends you a conference invitation that enables anonymous users to join the conference.
You try to join the conference.
In this scenario, you can join the instant message conference. However, you cannot join the audio or video conference.
To resolve this issue, install the following cumulative update:
2842627 Description of the cumulative update package for Lync 2010: July 2013