Symptoms
After you install the Microsoft Skype for Business Server 2015 CU4 (November 2016 Cumulative Update), the simultaneously ring feature only works intermittently in the Skype for Business client.
Cause
The issue occurs because the PSTN usages are not associated with the call correctly.
Resolution
To fix this issue, install the May 2019 cumulative update 6.0.9319.548 for Skype for Business Server 2015, Web Components Server.
