Consider the following scenario in a Microsoft Skype for Business Server 2019 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 July 2019 cumulative update 7.0.2046.123 for Skype for Business Server 2019, Front End Server and Edge Server.