ISDN line should be disconnected after idle-timeout expired once the Primary line becoms active. That is true if no interesting data going over the ISDN line.
So you can debug that by checking that idle-timeout is expired or not OR it is keep on resetting back to max..
Here is the url for that
http://www.cisco.com/warp/public/471/idle_timeout.html
Once the primary line becomes active, the idle-timeout on the isdn interface will count down towards 0. "sh dialer interface " will show that.
Once it becoms 0, "debug dialer" will report line disconnected because of idle-timeout expired..