Consider the following scenario in a Microsoft Skype for Business Server 2015 environment:
You are a federated Skype for Business user to User A.
The Busy Options with the Busy On Busy feature is enabled in User A's organization.
User A is already engaged in another communication stream.
You make a call to User A.
In this scenario, the call fails and you receive the following error message:
Call was not completed or has ended.
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 and the May 2019 cumulative update 6.0.9319.548 for Skype for Business Server 2015, Web Components Server.