No more monkey jumping in the bed, I mean the white-list method DOES give a better protection to my asterisk server. However, it has its main drawback, i.e., all users must be in the white-list.
It works with hostnames too. However it needs frequently(typ. 10 min.) running script to resolve the hostnames.
The white-list starts with the "host_" string and hostnames are "hided" within the comments.
See https://wiki.mikrotik.com/wiki/Use_host_names_in_firewall_rules