[Serusers] UA's behind the same NAT

Hi All I am trying to improve my proxy setup to force clients behind the same NAT device to connect each other directly. My setup is fairly complex, (rtpproxy / NAT Helper, PSTN gateways, different peers etc) but it's loosely based on the OnSip.org setup. The current config works correctly for all clients, whether behind a NAT or not, but I'd prefer not to have to use RTP proxy to allow UA's on the same nat to contact each other. I've seen the document from the AVPops module, but I can't get the example to work. I think the problem is WHERE I put the avpops config. As this works on FWD, I think it's possible. Has anyone got this working? Noel