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SIP PBX functions
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SIP PBX functions
There's an IETF Internet Draft that describes a lot of common PBX functions and how these could be implemented in SIP.
From the Introduction:
This document gives examples of Session Initiation Protocol (SIP) services. This covers most features offered in so-called IP Centrex offerings from local exchange carriers and PBX (Private Branch Exchange) features. Most of the services shown in this document are implemented in the SIP User Agents, although some require the assistance of a SIP Proxy. Some require some extensions to SIP including the REFER, SUBSCRIBE, and NOTIFY methods and the Replaces and Join headers. These features are not intended to be an exhaustive set, but rather show implementations of common features likely to be implemented on SIP IP telephones in a business environment.
This document provides example call flows detailing a SIP implementation of the following traditional telephony services:
- Call Hold
- Music on Hold
- Unattended Transfer
- Consultation Hold
- Unconditional Call Forwarding
- Attended Transfer
- No Answer Call Forwarding
- Busy Call Forwarding
- Single-Line Extension
- 3-way Call
- Incoming Call Screening
- Call Pickup
- Call Park
- Outgoing Call Screening
- Automatic Redial
- Click to Dial
The call flows shown in this document were developed in the design of a SIP IP communications network. They represent an example set of so-called IP Centrex services or PBX services.
It is the hope of the authors that this document will be useful for SIP implementers, designers, and protocol researchers alike and will help further the goal of a standard implementation of RFC 3261 
These flows represent carefully checked and working group reviewed scenarios of SIP service examples as a companion to the specifications.
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