This issue occurs in the following scenario:
- You don't deploy an edge server in a Microsoft Lync Server 2013 environment.
- You use HTTP proxy in Microsoft Skype for Business installation that has the December 8, 2015 security update installed.
- You allow the Internet connection only through HTTP proxy.
- You try to anonymously join a non-federated Lync audio and video (AV) meeting in the client.
This issue occurs because there is a regression in the December 8, 2015 security update that prevents Microsoft Lync 2013 (Skype for Business) from building a media connection with Lync Server. Therefore, the client can't connect to the server to join the meeting.