can I do without a loopback gw in this case? I mean when creating routes...

Actually I did the same (at least I think so) for ip6010 and trixbox pbx. But the calls between ip6010 and ip6010 are recognized as internal. Meanwhile the calls between ip6010 and trixbox are recognized as external, though they all use the same "intern" trunk object ("Route-Node External Calls to" option).