[RTPproxy Users] Kamilio integration with rtpproxy

[Joan]

Hello, I've been trying to setup a testing scenario to offer voip services.
At this point I've a phone behind a nat, and I am trying to
communicate to the openser, that in turns forwards the calls to an
asterisk server.
Before nating the phone and starting with the rtpproxy everything was
working properly.
The current scenario would be:
    IP Phone (192.168.2.50) -> NAT server masking as 192.168.253.12)
-> Kamailio (192.168.253.98) -> Asterisk (192.168.253.2)

The working scenario was:
    IP Phone 192.168.253.25 -> Kamailio (192.168.253.98) -> Asterisk
(192.168.253.2)

I've configured everthing in kamailio as the examples I saw, but I
suspect there's something wrong with my rtpproxy (I'm using
rtpproxy-1.2.alpha.20081104)
The fact is that when I sniff the traffic in localhost, having started
rtpproxy as "rtpproxy -s udp:localhost 15000 -f -u openser -l
192.168.253.98 &"

###################################################################################
test:~# ngrep -lqtt -W byline port 15000 -d lo
interface: lo (127.0.0.0/255.0.0.0)
filter: (ip or ip6) and ( port 15000 )

U 2008/11/20 16:46:13.466299 127.0.0.1:59654 -> 127.0.0.1:15000
19694_4 L e9f2a818-d99e70b5 at 192.168.2.50 192.168.253.2 17790
4e178bc90eb3861o0;1 as64b2e8a6;1

U 2008/11/20 16:46:13.466376 127.0.0.1:15000 -> 127.0.0.1:59654
19694_4 0 192.168.253.98
###################################################################################

I see that rtpproxy is returning 0 to the openser doesn't like it and
it says "ERROR:nathelper:force_rtp_proxy2_f: incorrect port 0 in reply
from rtp proxy"

Do i have a broken rtpproxy, or it's a matter of configuration?