asterisk users - Feb 2006 - Zap Auto disconnect after xx seconds of silence

I've got lines coming in from a legacy system (into FXO ports) which 
does not give any disconnect notification.  Folks familiar with the 
system say that I can buy or build a device which will listen for so 
many seconds of dead air and then automaticly send a disconnect signal 
to free up any hung channels.

This seems like something that could be done in software with Asterisk?

Right now the main problem is that these users join a MeetMe meeting and 
  don't free the line after they hang up.


Phil