Im using SIP proxy with rtpproxy behind a PIX firewall, I can use ATAs with NAT without problems to make and receive calls, but when I use another type of telephone like grandstream I only can make calls, the ports which I have open in my firewall are 5060 tcp/udp. I dont know if its because the rtpproxy/ser server cant be behind a firewall or just need to open another ports of its just a bad configuration.