Hello, try this out:
open your "Settings" window, click on the "devices" tab and then click on the "Advanced VoIP Account Settings..." Button. Now you can check "Enable end-of-call-tone detection during playback".
This will work if your telephone system has an end-of-call-tone. It's the tone if you have a call with someone and the other side hangs-up and you hear the "beep-beep..." after hang-up!
Maybe this will help you! I can't try this out now, maybe someone can post here his result?
Thanks
Arthur