This issue occurs when you receive multiple PSTN conversation calls in a Microsoft Lync 2013 (Microsoft Skype for Business) SDK application that has the ConversationWindow.Handle property read.
Cause
This issue occurs because PSTN conversation calls don't have a conversation ID to generate a unique handle value for each conversation window.
Resolution
To fix this issue, install the September 8, 2015 security update (KB3085500) for Lync 2013 (Skype for Business).
Note Lync 2013 was upgraded to Skype for Business in April 2015.