If i understand you correctly, you want to disconnect the internet (ISDN link) when there is no traffic i.e. idle link
If yes, then you need to configure idle-timeout on the ISDN interface;
eg;
dialer idle-timeout 600 <---- Idle timeout 10 mins before disconnecting a call
http://www.cisco.com/warp/public/471/idle_timeout.html
HTH
R/Yusuf