"User cannot be reached" error when you add a user through Skype Call in an ACP conference in Skype for Business 2016

Symptoms

In an Audio Conferencing Providers (ACP) conference in Microsoft Skype for Business 2016, when you try to add a user by selecting the Skype Call option in the call, you receive an error message that states that "The user cannot be reached."

Resolution

To fix this issue, install the June 4, 2019, update (KB4464576) for Microsoft Skype for Business 2016.