The dahdi drivers and binaries are needed to use popular digital and
analogue telephony equipment and are typically installed alongside the
asterisk PBX (although these are not required for asterisk if you are
not using any hardware to interface with the POTS/PSTN system).  Note
that if you change your kernel after installing any dahdi components,
you'll have to re-compile dahdi-linux (a simple re-install will not
suffice), but may be able to keep your existing dahdi-tools package.

After installing dahdi-tools, all the modules for digium telephony cards
will be blacklisted in /etc/modprobe.d/dahdi.blacklist.conf - this is 
intentional to allow you to load modules in a specific order in rc.local.

This requires dahdi-linux.