Strange error

Hello, I'm trying to use siproxd to fix the voip here at the company.
Here's the deal, we had a Windows 2003 Server running as a router and 
firewall, and we decided to change to a Gentoo Linux box, running 
iptables and Kernel 2.6.20-8, and sudenly, the voip fone start working in 
a very peculiar way.
I'm in brazil, if I call from the voip fone, to a brazilian fone, it 
works just fine, but if we call from a norwegian telefone to the voip, it 
makes the connection(rings), but no audio can be heard on any side. :O
Fiddling a little with tcpdump, I think the error was the ip header that 
the Linux box is actually sending Telio(the norwegian guys that gave us 
the voip box), so I was trying to use siproxd to correct this.
Now, if I run the siproxd, and from an external server try to connect to 
the host at port 5060, it works just fine, the logs show a connection, 
but when I use the voip phone(or dial to it), nothing happens at the 
siproxd side, like if it wasn't even seeing the call, here's a tcpdump 
for an incomingo connection (got on the firewall)

tcpdump udp and port 5060 or portrange 16300-16700 -i eth1
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 96 bytes
12:08:57.412303 IP AAA.BBB.CCC.DDD.1182 > aaa.bbb.ccc.ddd.5060: SIP, 
length: 328
12:08:57.645884 IP aaa.bbb.ccc.ddd.5060 > AAA.BBB.CCC.DDD.1182: SIP, 
length: 376
12:09:01.476811 IP aaa.bbb.ccc.ddd.5060 > 
AAA.BBB.CCC.DDD.1182:85.119.136.102 SIP, length: 1133
12:09:01.489918 IP AAA.BBB.CCC.DDD.1182 > aaa.bbb.ccc.ddd.5060: SIP, 
length: 520
12:09:01.502569 IP AAA.BBB.CCC.DDD.1182 > aaa.bbb.ccc.ddd.5060: SIP, 
length: 544
(This is where it started ringing)

where AAA.BBB.CCC.DDD is the firewall IP, and aaa.bbb.ccc.ddd is the SIP 
provider IP.

Can anybody please help me?
I'm getting desperate over this.

Thanks.

Mateus


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