I will hopefully get the traces from another user, who owns both: a VTO and a VTH.
I am afraid, there is no manual (as usual). It's easy to get developers (coders), but hard to get them to create manuals for their programmings.
I did not try it by myself, but I guess "Active Reg." means active sip registrations on the local sip server. So if you register you VTH sip at the VTO's sip server, you should have a entry for it there.