Symptoms
In Microsoft Skype for Business Server 2015, consider the following scenario:
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User A is a Skype for Business Online user who is using on-premises PSTN connectivity via on-premises call handling (OPCH).
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User B is a PSTN caller who is calling user A.
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User B ends the call before user A answers the call.
In this situation, the Skype for Business client still rings even after the call is ended.
Resolution
To fix this issue, install the July 2019 cumulative update 6.0.9319.559 for Skype for Business Server 2015, Front End Server and Edge Server.