Consider the following scenario in a Microsoft Skype for Business Server 2015 environment:
You set up simultaneous ring to your delegate user A.
User A also sets up simultaneous ring to a delegate user B.
You receive an audio conference invitation.
In this scenario, both user A and user B receive the audio conference invitation.
If you have many delegates and your delegates also have delegates, the audio conference invitation will be sent to all the delegates due to simultaneous ring. This can cause the Skype for Business Server Front-End service (RTCSrv.exe) to exhaust a lot of memory.
To fix this issue, install the May 2019 cumulative update 6.0.9319.548 for Skype for Business Server 2015, Front End Server and Edge Server.
After you apply this update, the audio conference invitation will be sent to your delegates only and not be sent to the delegates of your delegates.