The problem was that HylaFax, which runs as the user 'uucp' on CentOS, couldn't open /dev/FS0, because the permissions were wrong.  Since I didn't see any way to make FreeSwitch change the permissions, I added one line to the /etc/init.d/freeswitch file:

*** 46,51 ****
--- 46,52 ----
                RETVAL=$?
                echo
          [ $RETVAL -eq 0 ] && touch $LOCK_FILE;
+       (sleep 120; chgrp uucp /dev/FS*; chmod g+r /dev/FS*) &
        echo
          return $RETVAL
  }

-- 
Steve