[Serusers] Trouble with Record-Route

I have a situation where SER sits between two independent networks, one being an isolated network and the other being the rest of the Internet. The isolated network is also a PSTN gateway with only one switch on that network. SER has two physical interfaces, and uses force_send_socket and rewritehost to make sure things go out the right interfaces. The problem is when the called network hangs up first and generates a BYE* first, if SER didn't add a record-route, the called network switch bypasses SER and attempts to send directly to the calling IP address, which doesn't work because the two networks cannot be reached directly. If I add just one record-route in SER, the called network now knows how to get back to SER to tell it about the BYE, and SER passes that BYE on to the calling network. So far so good. Unfortunately, this also causes the called network to add Record-Route to responses it sends out, like 183/200 with its own IP address. Those reach the SER without problem, but SER doesn't add its own Record-Route to the reply, so when this reaches the calling network, they only have a Record-route that points to an IP address they cannot possibly reach. I tried a number of things to get around this, and all failed. 1. Tried to add my own Record-Route to the reply, but SER doesn't allow this. Won't even start. 2. Tried to remove the Record-Route from the reply, but SER also doesn't allow this. Won't even start. 3. Tried to add both IP addresses for the two interfaces to by using two Record-Routes at the INVITE. This doesn't fix the response messages, and actually causes looping behavior or "I'm terribly sorry" responses. (I tried this with and without the "enable_double_rr" parameter set and it didn't seem to make any difference.) By altering the order of the two Record-Route you are pretty-much guaranteed to confuse one party or the other as each needs the Record-Routes listed in the opposite order. 4. Tried some combinations of the above with loose_route() but the usual outcome is SER sending messages back to itself rather than to the right place and eventually the dreaded "I'm terribly sorry" or "481 Transaction" gets sent out, usually to nobody in particular. So, are there any working examples out there for a SER that has two network interfaces, each in its own isolated network that correctly route messages between the two sides? I need SER to not only add Record-Routes when appropriate but to strip them off or replace them as the replies pass back through and back to the initiator. The network isolation was a security thing, not a NAT thing (although private IP space is in use), but it is not possible for a message from one network to reach the other without passing through SER, and some commercial equipment I am working with ignores the source of a message and only sends the response to wherever the record-route(s) say it should go and if no record-route, it honors the Via, while other gear ignored both and only replies to the source of the message. *The problems affecting called-party BYEs also causes major grief when re-INVITEs occur, which fail to make it to the calling network (or more likely, the 200 OK replies fail to make it back), causing the call to die after 32 seconds. Any examples would be greatly appreciated.