if you have a different proxy name (as server name) than the domain name, you need to use the outbound proxy (OBP) feature on the client: - set as domain the SIP domain you want (like bif.net or baz.net) - set OBP the address of the server (like sip.foo.net).
By this, the requests will have in FROM/RURI the SIP domain, but they will be sent to the OBP server.
OK, so I guess my view of how things work has been tainted by the SIP UA I have (Linksys SPA942 and SPA3000).
There is no domain setting, at least not one that affects the user domain in sip transactions. The OBP setting in these devices appears to play double-duty as both the SIP user domain and also where to send requests. However, there are 684 things that can be tweaked on the box, so I may have missed it... but I don't think so.
Thanks, -mark