dialup:/etc# grep mgetty /etc/ttys
ttyc00 "/usr/local/sbin/mgetty" dialup on insecure
ttyc01 "/usr/local/sbin/mgetty" dialup on insecure
ttyc02 "/usr/local/sbin/mgetty" dialup on insecure
ttyc03 "/usr/local/sbin/mgetty" dialup on insecure
ttyc04 "/usr/local/sbin/mgetty" dialup on insecure
ttyc05 "/usr/local/sbin/mgetty" dialup on insecure
ttyc06 "/usr/local/sbin/mgetty" dialup on insecure
ttyc07 "/usr/local/sbin/mgetty" dialup on insecure
ttyc08 "/usr/local/sbin/mgetty" dialup on insecure
ttyc09 "/usr/local/sbin/mgetty" dialup on insecure
ttyc0a "/usr/local/sbin/mgetty" dialup on insecure
ttyc0b "/usr/local/sbin/mgetty" dialup on insecure
ttyc0c "/usr/local/sbin/mgetty" dialup on insecure
ttyc0d "/usr/local/sbin/mgetty" dialup on insecure
ttyc0e "/usr/local/sbin/mgetty" dialup on insecure
ttyc0f "/usr/local/sbin/mgetty" dialup on insecure
dialup:/etc#
insecure надо заменить на secure :) а ttyc0* на твои модемы
также погляди в rc.serial - для некоторых мультипортовок и прочих странных девайсов требуется отдельная инициализация