If you take a look at the long listing of the
telnetd entry in the
/usr/sbin/ directory, I expect that you will see it is a symbolic link to the
busybox binary . . . and the version of
busybox that is present in the modem/router has not been built to accept a
-a flag (when it is called as
telnetd).
ls -l /usr/sbin/telnetd will probably show something like --
lrwxrwxrwx 1 root root 14 Aug 16 2013 /usr/sbin/telnetd -> ../../bin/busybox
It does seem as if all means of access have been disabled, one way or another.