This issue occurs when you receive multiple PSTN conversation calls in a Microsoft Lync 2013 (Microsoft Skype for Business) SDK application that has theĀ ConversationWindow.Handle property read.


This issue occurs because PSTN conversation calls don't have a conversation ID to generate a unique handle value for each conversation window.


To fix this issue, install theĀ September 8, 2015 security update (KB3085500) for Lync 2013 (Skype for Business).

Note Lync 2013 was upgraded to Skype for Business in April 2015.

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