Voice chat and NAT issues

[Federico Tello Gentile]

I have 3 computers, all running Ubuntu 9.10 64 bits. I can make voice
calls between 2 of them, but neither of them can make or recieve voice
calls to the third one.

The calls only work between the ones that have a public IP address
(cablemodem). The other one is a notebook connected over WLAN to a
router with NAT.

On the wireless router side I enabled Universal Plug & Play, disabled
the firewall and forwarded all ports to 192.168.1.2 (the only computer
on the LAN).

On the Pidgin side I saw the network tab was displaying the internal IP
addess as the detected public IP address, so I added a STUN server and
now the correct public IP address is detected.

I have no TURN server set up (and I don't know if I need one or which
one to use).

There's no error message on either side. The conversation is established
for as long as I want, but both sides hear nothing and the incoming
sound level stays still. The mic level moves as I speak, but the other
side hears nothing.

I captured traffic and I see traffic between the remote IP and my
internal IP saying with this STUN Error Code: 487 Role Conflict

Any help would be great, for example what to look for in the wireshark
packet capture (SIP packages?) or how to filter relevant traffic.

Pidgin's debug window does not show any error or warning messages, only
info.

What are the minimun requirements to do voice calls behind NAT? Googling
I don't find many people having trouble with it.