We currently run a VoIP server using an upstream providers SIP proxy for our clients who are behind NAT. We now have the problem that we re ending the relationship with the upstream provider, and will no longer have access to their nice NAT proxy!
We've decided we should probably build one ourselves. We've done some fairly extensive googling on this subject and it seems to be you need to use OpenSIPs and MediaProxy to build one. Problem is documentation on the subject seems to vary between really vague and assuming lots of back knowledge, or really outdated and therefore useless.
I'm looking for a thorough guide or set of worked examples on how to get a NAT proxy for VoIP set up working from scratch. Any pointers to known god guides, or just outright step-by-step answers will be appreciated!
Update
To clarify, what I'm looking for is a guide to building the server that the phone talks to through the "Proxy" setting of most SIP phones.
Best Answer
I have used Siproxd in the past in these situations (several SIP endpoints, registering with an external SIP softswitch, and a non SIP-aware router in between), and it has worked very well. It's quite simple to install and set up. The project page has detailed installation and configuration documentation.
If you want to go really simple, and you are able to switch out what router you use, fire up a pfSense router. pfSense has a siproxd package available, which is a single-click install.