Symptoms
When a user joins a Skype for Business meeting of a different organization that's not federated with the user's organization, the Skype for Business client doesn't run anonymous fallback. In this situation, the meeting join process fails, and the user sees a blank conversation window.
Resolution
To fix this issue, install MS16-097: Description of the security update for Lync 2013 (Skype for Business): August 9, 2016 (KB3115431).Note Lync 2013 was upgraded to Skype for Business in April 2015.
