The problem is that “just disconnecting:” if the phone is off-line is not a typical end-state. Normally, you would pick “go to voicemail” or “hang up” if the extension is not available…
Now, you may be able to get something like this to work with VM-Blaster, but I don’t think we differentiate that closely on most calls.
If you really want this kind of non-standard behavior, I think you’ll need to write a new “from-somewhere” context to make this happen.