[Serusers] SER, rtpproxy and SIPclient

My SER is running behind NAT with rtpproxy. I was able to send and receive message via Windows Messenger. Both clients were outside my network and they were also behind NAT. Problem is, I can't communicate with a client (outside my network) from my internal network. Here is my iptables rules -A PREROUTING -p tcp -d 66.92.x.y --dport 5060 -j DNAT --to-destination 192.168.71.2 -A PREROUTING -p udp -d 66.92.x.y --dport 5060 -j DNAT --to-destination 192.168.71.2 -A POSTROUTING -s 192.168.71.0/24 -d 192.168.71.2 -p tcp --dport 5060 -j SNAT --to 192.168.71.1 -A POSTROUTING -s 192.168.71.0/24 -d 192.168.71.2 -p udp --dport 5060 -j SNAT --to 192.168.71.1 -A POSTROUTING -s 192.168.71.0/24 -o eth0 -j SNAT --to-source 66.92.x.y my rtpproxy is running without any argument. Would anybody please give me hint, how to overcome this problem. Thanks, Mohammad