Mmmm, don't know, sorry... my bet would be on Agent Timeout (and/or appropriate Max Wait Time Mode setting).
(my own setup involves my provider trying each of the incoming trunk accounts for a set amount of time, which means I never really hit the conditions I have set on my queue settings, being MWT equal to AT and MWTM=Loose) (and I don't use VM on my own server, so the above was made to force the incoming calls to /providers/ VM if queue times out)