We need a /dev/isdnctrl device. Removing "rm -f /dev/isdnctrl" in the spec file should fix this.
The spec file mentions that /dev/isdnctrl is obsolete. Is this no longer the case?
I just noticed that /dev/isdnctrl was tagged as obsolete because it was moved to the isdn4k-utils package. I agree that device files should not be in packages other than dev (that, and it's not in the isdn4k-utils package any more), so I'm adding the link back in.