Thanks for the reply Alex! I really appreciate it. I've been reading manuals & sip specs all day, and I think that I found my answer, and it really is an easy one. here's my own answer to my question, can you let me know if this sounds right? When a message is send from a user agent (on the public internet) to my gatweway & then to the proxy (on a private network), the gateway will add a via on to the message. that's how open SER (or any other proxy) can send a response message back to user agent on the public internet via the gateway. It's really the same thing as setting up a registrar, or any other kind of proxy/user agent that's going to communicate via a SBC of any kind. So, it that right? is that how the SIP via header is used? I understand the part about registrations. We're not currently doing that in our network, we're using all static IP addressing. I plan on using registrations in the future. the gateway that I'm using can act as a session border controller, but it can also act as a class 4/5 switch (TDM or IP). so it's some serious overkill for what I want to do here. :o) Thanks again for the reply, --Doug ----- Original Message ---- From: Alex Balashov <abalashov(a)evaristesys.com> To: Doug McLetchie <dougmcletchieatwork(a)yahoo.com> Cc: users(a)lists.openser.org Sent: Wednesday, February 6, 2008 1:23:51 AM Subject: Re: [OpenSER-Users] newbie questions Doug, Doug McLetchie wrote: OpenSER can certainly do what you are trying to accomplish. SIP routing is done by URI, which consists of a "user" part and a "domain" part. The "user" part is the number (or alphanumeric identification, in the case of pure-VoIP peering) and the "domain" part is the IP "place" at which the "user" part is reachable. When you route a call to some URI, what you are really saying is, "Here, domain, you must know what to do with this 'user' part - i.e. have reachability information for it (a SIP contact bound from a registration, for example)." A proxy by itself isn't enough. If you need to reach these PBXs, you clearly need to know how to reach them. This requires a SIP registrar somewhere, so that the PBXs can register against it and say, "Here, you can reach me at such and such IP and port." Or, I suppose, you can define these contacts statically with a database interface from the proxy, in which case you don't need to use a registrar. I don't know what this Big Expensive Gateway is, but if it's something like a Session Border Controller, it should be able to forward SIP REGISTER requests to your proxy/registrar. Or do they register against the gateway? Either way, you can perform this resolution with OpenSER. The problem you *might* run into is sending the same logical call leg back to the Gateway. Depending on what it is, it may not like that and perceive a call routing loop, because the call that went through it to the proxy is the "same" call (in terms of SIP Call-ID, and other things that make up a logical call "leg") that is now being sent back around to it. This problem is often solved with the introduction of a back-to-back user agent which can re-originate a different call leg. -- Alex Balashov Evariste Systems Web : http://www.evaristesys.com/ Tel : (+1) (678) 954-0670 Direct : (+1) (678) 954-0671 Mobile : (+1) (706) 338-8599 ____________________________________________________________________________________ Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ