When installing the XPhone Connect Client, the following message is displayed:
"The file "adxregistrator.exe" is not marked for installation."
This error message is not an XPhone-specific problem. Therefore the cause must be searched on the system itself.
When searching for the bug, you can also find some articles about other products, such as Salesforce for Outlook (see e.g.: https://help.salesforce.com/s/articleView?id=000336421&type=1).
In this case the following tool from Microsoft can help:
Uninstall XPhone using this tool and then restart the installation.
Attention: The required .NET Framework 4.8 for the XPhone Connect Client must be installed and all Windows updates for it must be applied (see https://learn.microsoft.com/en-us/dotnet/framework/migration-guide/how-to-determine-which-versions-are-installed)