[Serusers] how to forward a request to upper proxy???

[szj]

Hi:

    I have one question to get help from you! If UA call a
user by my SER Proxy, and  SER can't find the called user
in itself location, it will return 404"not found". But this is
not What I want. What I want is to let SER forward the
request except REGISTER to upper proxy??
  How should I configure SER to get the goal ???
   Thanks a lot for your help!

    Best Regards
 
Sun Zongjun