The PreCallDiagnostic tool is a fantastic tool to simulate and diagnose the Quality of the Audio stream for OCS
It simulates an audio call and provides information about Network MOS, Packet Loss and Jitter values

However : The tool does not always work as expected.

You might get this message saying:
There was an error starting the channel.  The following detailed error message might provide more information.
There was an error signing into the SIP server using the SIP URI you’ve configured.  Please double check that the SIP URI is correct for the logged-on user.  Also, this might indicate that the server might be temporarily down.

You double check the SIP URI but see no problem.

The reason why the tool might fail is that you can configure the SIP URI in the application but you cannot provide any credentials.
So what happens is that the tool uses the current Logged-in user credentials to authenticate to OCS

If you are not logged into the domain, chances are so that the credentials are invalid for OCS and then you can’t use this tool.
So make sure that you are logged into the domain or have logged into the cached domain profile when running this tool.

This means that for a consultant like me this tool cannot always be used in a flexible way.
I hope that Microsoft will update the tool and give the opportunity to provide credentials manually instead of just using the current credentials

Greetings,
Johan