prerm is in dahdi-dkms 1:2.5.0.1+dfsg-1ubuntu3.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 | #!/bin/sh
NAME=dahdi
PACKAGE_NAME=$NAME-dkms
CVERSION=`dpkg-query -W -f='${Version}' $PACKAGE_NAME | cut -d\: -f2`
case "$1" in
    remove|upgrade|purge)
		echo "Removing all DKMS Modules"
		dkms remove -m $NAME -v $CVERSION --all > /dev/null
		echo "Done."
	;;
    purge)
		delgroup --quiet $NAME
        ;;
esac
 |