[SR-Users] RTPENGINE - RTP Proxy

Hello all, I'm currently doing some experiments with RTPENGINE and I need some help configuring it. First scenario i'm under NAT and Kamailio receives an INVITE with a private address, rewrites (c) line on SDP with it's own IP address and relays the message to the destination endpoint. After that the originator receives the 183 Session Progress with kamailio's IP as (c) parameter. After receiving the first RTP packet from the originator the media streams flow normally and I have sound on both sides including ringing. Second scenario I use different equipment that is not under NAT, but the remaining scenario is the same, except the originator doesn't send an RTP packet after the 183 Session Progress. However, in this case, no RTP stream is relayed to the originator and as a consequence it doesn't hear a ring or voice when the call is answered. As i see it, it looks like Kamailio is on passive mode to the A-Leg of the call and waits for a RTP packet to establish its stream. Is this correct? If so, how do i change this behaviour? Cheers