Consider the following scenario:
You join a Skype meeting in a Microsoft Skype for Business Server 2015 environment.
You invite a Public Switched Telephone Network (PSTN) number to the meeting.
You click the End Meeting button before the PSTN callee takes the call.
In this scenario, the call continues to ring on the PSTN phone.
This issue occurs because the Audio Video Multipoint Conferencing Unit (AVMCU) cannot send the Cancel request to the Mediation Server.
To fix this issue, install the May 2017 cumulative update 6.0.9319.281 for Skype for Business Server 2015, Front End server and Edge server.