This issue occurs when you have multiple SIP connections that are made to the same destination (same IP and port) in Microsoft Lync 2013 (Microsoft Skype for Business).
When this issue occurs, the clients are disconnected repeatedly. That may cause some problems such as signing in, instant messaging (IM) and presence delayed or intermittent.
To fix this issue, install the June 7, 2016, update (KB3115033) for Lync 2013 (Skype for Business).
Note Lync 2013 was upgraded to Skype for Business in April 2015.