You cannot leave a voice mail message until the Skype for Business Front-End service is restarted in a Microsoft Skype for Business Server 2015 environment. Additionally, the following event is logged on the Microsoft Skype for Business 2015 Front End server.
Time: Date/Time
ID: 30210
Level: Warning
Source: LS Applications Module
Machine: SERVER Name
Message: Lync Server application is not responding and is being disconnected.
Application Uri 'http://www.microsoft.com/LCS/ExumRouting' Cause: This can occur if the managed application is slow to respond to messages or if the server is under heavy load or the application has become unresponsive.
Resolution:
If the server load is too heavy, consider getting a faster machine or hosting fewer users on the server. If the application is slow, contact the author of the application.