Assume that you have multiple conversation tabs opened in Microsoft Skype for Business 2015 (Lync 2013), and you use the Windows hotkey (Windows + F4) to mute or unmute the current audio. In this situation, unread prompts on previous audio conversation windows are triggered. Additionally, you will see a generic message like "call is already ended". Even if you dismiss the prompt message, it will repeat when you use the hotkey.
To fix this issue, install the December 6, 2016, update (KB3127976) for Skype for Business 2015 (Lync 2013).
Note Lync 2013 was upgraded to Skype for Business in April 2015.