Down state won't make a button in FOP2 be marked as busy. FOP2 is doing the right thing. Does that not mean that there is a bug/problem either on your setup, sometimes there is a brief moment of a channel being in down state before the call is set up, it seems in your case is taking a little to long to be marked as such. In general, what I tell to users is that trunk buttons will show active/bridged channels/calls on them only. If you can make Asterisk signal the channel as up, it will show as you expect.
Best regards,
