[Serusers] Rewriting From header fields

Hi, after reading through the archive of this list I feel like I need to come up with an old issue once again, since I have not found a suitable solution for my setup. I have to user agents. #1 is a registered iptel user and #2 is registered with say "ipfon.org". UA #2 is only capable of numerical adressing within its own realm (such as a Grandstream phone). UA #1 is known to the world as 12345678(a)iptel.org and can be reached from UA #2 through its proxy by inviting 39312345678(a)ipfon.org (i.e. the user can dial 39312345678 on the Grandstream to reach UA #1). It would be terrific to submit a working callback address for UA #1 to UA #2. This callback address however, needs to be within the called party's realm, I believe (the Grandstream has comes with a dialpad only). Unlike some pstn gateways the Grandstream does not parse the Remote-Party-ID header field. Is there another header field to address this issue? If not, I have no idea how this can be solved but by rewriting the From header such as UA #1 Proxy UA #2 INVITE --> "REWRITE" --> INVITE From: 12345678(a)iptel.org From: 39312345678(a)ipfon.org and keeping track of any subsequent messages. How would you approach this problem? I read through tmemo-jiri-sat.txt as this memo adresses the potential problems and suggest an implementation. Has anybody started to implement the suggested solution? Thilo