There are a few different types of SIP servers. Depending on the application, you can use one or all of them in your solution. OpenSIPS can behave as a proxy, redirect, B2BUA, or Registrar server.
In the SIP proxy mode, all SIP signaling goes through the SIP proxy. This behavior will help in processes such as billing and is, by far, the most common choice. The drawback is the overhead caused by the server in the middle of all the SIP communications during the session establishment. Regardless of the SIP server role, the RTP packets will go directly from one endpoint to another even if the server is working as a SIP proxy.
The SIP proxy can operate in the SIP redirect mode. In this mode, the SIP server is very scalable because it doesn't keep the state of the transactions. Just after the initial INVITE, it replies to the UAC with a 302 Moved Temporarily and is removed from the SIP dialog. In this mode, a SIP proxy, even with very few resources, can forward millions of calls per hour. It is normally used when you need high scalability but don't need to bill the calls.
The server can also work as a Back-to-Back User Agent (B2BUA). B2BUAs are normally applied to hide the topology of the network. They are also useful to support buggy clients unable to route SIP requests correctly based on record routing. Many PBX systems such as Asterisk, FreeSwitch, Yate, and others work as B2BUAs.
