[Serusers] serving a domain with two ser proxies on a private network and the internet -- RFC

Hi, I do have two ser servers (0.8.14) and a pstn gateway (*) and a rather complex setup to build. One ser server and the pstn gateway are inside a private network that is shielded from the internet by a cisco pix firewall. This ser server should act as a sip registrar and outbound proxy for all clients that are inside the private network and should furthermore handle incoming calls from the pstn gateway. The other ser is on the public internet (public ip) and the firewall that secures the private network allows connection from the internal private ser server to the external one and vice versa (translating the private ip from the internal ser to a public one). Clients from the internal network cannot and must not traverse the firewall. I want to configure the internal ser in a way that it acts as a sip and a rtp registrar and outbound proxy (rtp with mediaproxy for video if possible). All calls from internal clients and the pstn gateway should be accepted by the internal ser and either be handled on that server directly (destination also on the internal network) or be forwarded to the external server to be handled there. As stated above, all sip messages and rtp streams can only leave the internal network via the internal ser server. Calls from external clients to internal clients should be accepted from the external ser server and be forwarded to the internal one with the rtp streams and sip messages being proxied by the external ser server. Incoming sip and rtp traffic for internal clients is only allowed from the external ser to the internal ser by the firewall. Both sers should serve the same domain as a registrar to allow clients to be mobile and either connect to the internal ser if on site or to the external ser if at home or somewhere else. There are two dns servers, one internal on the private network and one external, the internal one (used by all internal clients) replies with the private ip of the internal ser and the external (public dns server) replies with the public ip of the external ser when asked for sip.xyz.com. The external ser has already a running version of mediaproxy on it and both ser servers authenticate their users by a common database on another server. Finally it would be great if presence would work across the two servers as well as having accounting on a central database. Running a common database for both sers on another server would be possible if required. Has anybody a similar setup or can provide me with some ideas et c.? Thanks and have fun Morten