Why does the VOIP/SIP application that is using the dialer not work properly in the Knox container?

Knox Workspace

A VOIP application that uses the Android dialer is not working properly in the Knox container.
In the Google's Multi User Framework concept, only the main user can make calls. The Knox container, which is an additional user on the device, cannot have its own dialer application.
Use an application that does not rely on the Android dialer but instead its own custom one.