This issue occurs when you do a PC-to-PC call in Microsoft Lync 2013 (Microsoft Skype for Business) from an external network. When this issue occurs, media setup may fail. Additionally, all media diagnostics information is missing when you check ms-client-diagnostics in the client log Session Initiation Protocol SIP) BYE message.
To fix this issue, install the December 8, 2015, security update (KB3114351) for Lync 2013 (Skype for Business).
Note Lync 2013 was upgraded to Skype for Business in April 2015.