Symptoms
When you receive a public switched telephone network (PSTN) call in Microsoft Skype for Business 2016, the call control window flashes.
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 December 6, 2016, update (KB3127980) for Skype for Business 2016.
