A SIP server is the main component of our IP 3CX, and mainly deals with the management of all SIP calls in the network. A SIP server is also referred to as a SIP proxy or a registrar. Although the SIP server can be considered the most important part of a SIP-based IP-3CX phone system, it only handles or manages sessions; more specifically, a SIP server can:
Set up a session between two (or more) endpoints (an audio conference would have more than two endpoints)
Negotiate the media parameters and specifications for the session for each endpoint using the SDP protocol
Adjust the media parameters and specifications of a session during the session (putting a call on hold, for example)
Substituting one endpoint with a new endpoint (call transfer)
Terminate a session
The SIP server does not actually transmit or receive any media – this is done by the media server in using the RTP protocol. Within the context of an IP-3CX environment, it is almost always true that the SIP server and its Media server companion reside on the same machine. Do keep in mind, however, that very-high-volume SIP servers (such as a large VoIP provider, for example), may separate their media server to a different machine to better handle the workload, and could also possibly distribute the load to multiple media servers.