Consider the following scenario:
You deploy a proxy server that only support basic authentication in an organization. There is no DNS resolvers for Internet server in the organization.
A Microsoft Lync 2013 user (user A) in the organization receives a meeting invitation from another user (user B). User B is in an organization that is not federated with user A’s organization.
User A tries to join the meeting.
Note To do this, user A must access Internet through the proxy server.
In this scenario, Lync 2013 keeps trying to connect to the meeting instead of prompting user A for credentials.
To resolve this issue, install the following update:
2881070 August 2014 update for Lync 2013 (KB2881070)