[Serusers] Can SER authenticate to an "uplink" proxy?

Hello, after reading some documents on iptel, googling and going through the mailing list I still do not know how to set up the following configuration - or if such a configuration is possible with SER at all: +---------+ +-----+ +-------+ |sip-phone|----| SER |----|sipgate|--PSTN... +---------+ +-----+ +-------+ (...) I have some sip-phones which should register at "my" SER (this is the easy part). I also have an account at sipgate (and other GW-providers) which I would like to use for incoming and outgoing calls. How can I configure my SER so that it REGISTERs at sipgate, so that sipgate will send INVITEs to my SER when there is an incoming call? How can I configure SER so that is authenticates against sipgate when setting up an ougoing call? The final setup should allow me to make "internal" calls between the sip-phones, receive incoming calls from some "public" GW-providers and use "least-cost-routing" for external calls (use cheapest GW depending on destination). I believe that such a configuration is not that exotic, but I could not find a usable sample... Thank you Dietmar Wessel