Table 2.6 IP PBX and Centrex Service Examples Using SIPCall Hold
Consultation Hold
Unattended Transfer
Call Forwarding, Unconditional
Call Forwarding on Busy
Call Forwarding on No Answer
Three-Way Conference
Single Line Extension
Find-Me
Incoming Call Screening
Outgoing Call Screening
Secondary Number - In
Secondary Number - Out
Do Not Disturb
Call WaitingPBX systems have additional, application area–specific features that we do
not list here for brevity, and also because most people rarely make use of them.
Such features are mostly proprietary.SIP Service Creation—Telephony-Style
The wide range of possibilities for new services is matched by the ease of ser-
vice creation and deployment [14]. Simple services can even be developed by
end users.
■■ Call Processing Language (CPL)[15] is mainly intended to be used by
nontrusted end users to upload their services on SIP servers. XML
scripts created by end users can be uploaded to SIP servers for call
setup in a secure execution environment.
■■ SIP Common Gateway Interface (CGI)[16] is analogous to the Common
Gateway Interface (CGI) used for web server access to databases. Com-
plex services can be programmed under control of network administra-
tors using SIP CGI.26 Chapter 2