SIP

In this article, SIP would be a simple protocol. But SIP can be a base protocol for IMS. It is very complex.

Overview

The Session Initiation Protocol (SIP) is a communications protocol for signaling and controlling multimedia communication sessions.
In this article, SIP would be a simple protocol. But SIP can be a base protocol for IMS. It is very complex. Normally IP-PBX can be a SIP server. Sometimes service server also can support server mode. If you want to integrate with other equipment, you need to consider supporting SIP. In the world, there are number of SIP devices.

Protocol

SIPSession Initiation ProtocolRFC 3261
SDPSession Description ProtocolRFC 4566
RTPReal-time Transport ProtocolRFC 1889, 3550
RTCPRTP Control ProtocolRFC 3550

Transport

TCPTCP-Interleaved
UDPunicast, multicast

Mode

Caller mode

Client call to peer.

Callee mode

Client can have a call from peer.

Reference

Ref https://en.wikipedia.org/wiki/Session_Initiation_Protocol