User guide

4. The recipe
This section summarizes the basic concepts behind VoIP. Understanding each of them will be very
useful when you start configuring any software related to IP telephony. Although VoIP is a vast
knowledge area, we have carefully selected a few essential concepts. This section provides a basic but
still solid understanding of what you need to know to get started and build your first telephone system.
4.1 PBX
A PBX or a PABX is a non self-explanatory technical acronym that stands for
Private (Automatic) Branch Exchange. In simple words, the most common
use of a PBX is to share one or several telephone lines with multiple users.
A PBX sits between the telephone lines and several phones (voice terminals). A PBX has the ability to
redirect incoming calls to a given phone, or to allow phones to choose a specific telephone line to place
a phone call. In the same way that an Internet router is responsible of redirecting data packets from a
source to one or more destinations, a PBX routes “phone calls.”
The concept of “private” in the acronym PBX means that the owner of the system manages the unit
and decides how to share the external phone lines with its users.
A PBX does not only allow you to share a telephone line among several users but also provides value-
added services such as transferring calls, three way calling,
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voice mail (or voice to e-mail),
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Interactive Voice Response (IVR)
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services, etc.
A PBX can be very useful in several different scenarios. For example, in developing regions, access to
the telephony network often imply a visit to a calling point (private phone booth) or a long walk to a
Telecentre. A common situation is that only one telephone line is available in an office building or in a
rural area but several organizations, located in the neighbourhood, want to be able to receive and place
phone calls. A PBX permits organizations and individuals in a rural village to connect and share a single
available phone line.
10. Three way calling is the possibility of having more than two people talking in the same conversation.
11. A voice to e-mail service allows one to record a message (like in an answering machine) and forward it to an e-mail account.
12. An Interactive Voice Response system allows the telephone caller to select an option from a voice menu by pressing a
number on the telephone keypad.
Page 9 TRICALCAR | www.wilac.net/tricalcar – Version: February 2008