that is correct. when the call is over in cisco voip world the system clears the call. if you watch the display you will see this. on a tdm system it gives you a busy signal to remind you to hang up the phone because you are tying up resources. I am not sure and would have to do a test but I believe that the ata should give some sorta of signal to let the user know that the call is over. either a busy signal or a new dial tone.
hope this helps
scott