[Asterisk-Users] X-Lite/Firefly behind NAT connecting to Asterisk not receiving RTP

[Kevin P. Fleming]

I have an asterisk server up and running, using Firefly in IAX mode 
works great, even with Firefly behind a NAT (as expected, since IAX 
works really well with NAT).

Now I'm trying to get X-Lite and/or Firefly to work in SIP mode from 
behind the NAT, and I can't seem to get there.

At this point, the phone will successfully register with Asterisk, and 
the Asterisk qualify messages get delivered to the phone. I can initiate 
a call with the phone, or Asterisk can send a call to the phone. In 
either case, the RTP traffic does not get accepted by the NAT in front 
of the phone, because X-Lite (and Firefly) are not sending the _first_ 
RTP packet to get the conversation started.

In X-Lite I have specified the Asterisk server as an "outbound proxy", 
and its logs shows that setting has taken effect. In Firefly there is no 
means to make a similar setting. STUN does not make any difference, 
because Asterisk is already sending the RTP packets to the correct IP 
address, the firewall is just not allowing them through because it 
doesn't know the client (phone) is expecting them.

Obviously I can reconfigure the firewall to open up ports for RTP, and 
port-forward them to the computer that X-Lite/Firefly is running on, but 
that's not going to work with two phones behind the NAT :-(

I just don't understand what I'm missing here... aren't SIP clients that 
know they are behind NAT supposed to initiate the RTP conversation 
themselves, so their NAT firewall will know to accept the RTP packets 
coming from Asterisk?