The use flag dahdi require crc_ccitt_table to be part of the kernel (direct or as module) (CONFIG_CRC_CCITT=m). This can be done using a chkconfig in linux-info eclass
That is a net-misc/dahdi dependency which should not be duplicated in net-misc/asterisk.
As per our conversation on IRC, this dependency is expressed on the net-misc/dahdi package which actually builds the kernel modules.