Call Processing Server – otherwise known as an IP PBX, is the heart of a VoIP phone system, terminating all VoIP control connections. VoIP communications require a signaling mechanism for call establishment, known as control traffic, and actual voice traffic, known as voice stream or VoIP payload. With the exception of routed voice traffic to another call processing server, conferencing functionality and music-on-hold, call processing servers do not handle VoIP payload (which is the RTP stream carrying voice itself) traffic, which usually flows between VoIP terminals (end-devices), such as VoIP Phones.
VoIP control traffic follows the client-server model, with VoIP terminals, including messaging servers (which hold voice-mail messages), are the clients that communicate to the call processing servers. However, VoIP payload flows in a peer-to-peer fashion – from every VoIP terminal to every other VoIP terminal. In this case, the VoIP terminals determine traffic flows and the call processing servers negotiate those flows within the control messages.
Call processing servers are usually software-based, but may also be based on router platforms or developed as a dedicated appliance. A single server, cluster of servers, or a server farm with distributed functionality may provide call processing functionality.