Consider the follow scenario:
You sign in to Microsoft Lync 2010 in a Lync Server 2010 environment that has the cumulative update for Lync Server 2010 that was released in February 2012 installed.
Anonymous user access is enabled in the environment.
You create a Lync Online meeting.
You invite a Lync 2010 user to join the meeting. The user's Lync 2010 client has the cumulative update for Lync 2010 that was released in April 2011 or a later cumulative update installed. Additionally, the user is in a Lync Server 2010 organization that is not federated with your organization.
In this scenario, the user cannot join the meeting.
This issue occurs because a Lync 2010 user who is not in a federated organization is not recognized as an anonymous user. Therefore, the user's client is disconnected from the Lync Server 2010 server.
To resolve this issue, install the following cumulative update:
2701664 Description of the cumulative update package for Lync 2010: June 2012