asterisk users - Nov 2009 - Sip phones on localnet AND outside localnet problem

Hi list

I am having trouble getting asterisk to perceive the firewall's ip address
as outside localnet (setting in sip.conf). The situation is this:
- phones inside lan work fine when localnet is set to 192.168.0.0/255.255.255.0
- phones outside the lan can't ack the invite from asterisk because asterisk
perceives the outside phone to originate from localnet as DNAT is done with
iptables and the incoming ip address is rewritten (i'm assuiming) to the
firewall's internal ip address (192.168.0.20) (so from and contact are
192.168.0.21, this is the asterisk box's private ip)
- If i set localnet to 10.0.0.0/255 asterisk puts the externip in the from and
contact fields and it works fine with an external phone
- I haven't tested it yet, but i don't think a sip phone in the lan with
the asterisk box will work if an incorrect localnet is set!

Can anyone help or offer a workaround for this to allow sip phones in the lan as
well as outside to work? Is it possible to mangle the packets with iptables in
postrouting so that they don't have the firewall's ip address when
received by asterisk?

Is there a setting in sip.conf that can make the firewall ip appear as non-local
even though it is in the localnet setting?

Thanks for any help!

Marcus