When a user joins a Skype for Business meeting of a different organization that is not federated with the user's organization, the Skype for Business client fails to perform anonymous fallback. In this situation, the meeting join process fails and the user sees a blank conversation window.
This issue occurs because of a change that was made in the July 5, 2016, update (KB3115261) for Lync 2013 (Skype for Business) and in some Current Channel Click-to-Run builds of Skype for Business 2016.
Lync 2013 (Skype for Business)
This issue is fixed for Lync 2013 (Skype for Business) MSI-based clients. For more information, see the following Microsoft Knowledge Base article: