If you are running a large or complex VoIP environment, you need a proxy server. One of the most powerful SIP proxy servers ever developed was known as Sip Express Router (SER). SER spawned several spin-off projects, including OpenSER, OpenSIPS, Kamailio, and the Sip Router Project. They are all based on the same proven, powerful technology, and are in use all over the world effortlessly handling massive VoIP operations (carrier and enterprise environments serving hundreds of thousands of users).
We use this technology to expand the scope of our solutions, assuring essentially no practical limits to the scalability of our systems. If you have more than 250 telephones in your enterprise, this technology will allow you to expand your system seamlessly up to tens of thousands of phones (or more). Redundancy, high-availability and continuity are also benefits that will be realized from deployment of proxy and registrar servers.
In the above diagram VoIP call setup and call routing are directed through and maintained by the (A) OpenSIP server. All related voice applications are provided by a (B) media server, such as Conference Bridge. The (C) Session Border controller provides security for all the ingress and egress of VoIP calls into the private LAN, private WAN and Internet (D).