You need to have the interface up, otherwise it is not allowed to dial.
Do you have link1 set on ippp0 ? If not, it uses "flatline" mode and always
tries to stay connected.

If you have link1 set, but it does not disconnect, there either is traffic
that prevents it from reaching the idle timeout (quite possible with todays
steady stream of scans) or your isdnd configuration is wrong.

Martin