What is PBX- By: Musarrat Sheikh

Description : PBX is the short form of Private Branch Exchange which is a small telephone switch that is owned by a company or an organisation. These companies purchase PBX's in order to reduce the total number of telephone lines they will need to lease from a telephone company. Without this machine a company will need to lease one telephone line for each of the employees with a telephone. With this machine, a company will only need to lease as many lines as the maximum number of employees who will be making outside calls at one time which is usually around 10 percent of the number of extensions.
This machine wires all telephones and when someone picks up the telephone and dials the outside access code which is usually 9, the Private Branch exchange connects the employee to an outside line which is often referred to as a truck call which in some cases is incorrect. The are two common Private Branch exchange systems that are currently in the market and these are Avaya's Definity Series also known as System 75 and System 85 and Northern Telecom's Meridian Series. There are however other Private Branch Exchange system manufacturers that will include NEC, Siemens and Mitel. There are also several complete software based Private Branch Exchange systems that are available as open source software.
These packages are provided at no cost and also have large support networks of existing users who provide product support. Examples of the various types of packages that are available in the market will include:
Asterisk - which runs on BSD Unix, Linux as well as Mac OSX and provides users with all of the features they expect from a top of the line hardware based Private Branch Exchange which will include IVR capabilities. It also supports VoIP in many protocols and also interoperates with almost all standards based telephony equipment that use relatively inexpensive hardware.
GNU Bayonne - This is a telephony server that offers free scalable, media independent software environment for development as well as deployment of telephony solutions for using with current and next generation telephone networks. It supports IVR scripting as well as performs script driven IVR applications written in its native scripting language as well as access conversion and playing of audio from remote URL's.
FreeSwitch - this is designed to take advantage of as many existing software libraries as possible. It also makes it possible to build an open source VoIP switching platform as well as unite various technologies that will include SIP, H.323, IAX2, LDAP, XMPP among others. It can also be used to interface with other open source systems like GNU Bayonne, Asterisk and OpenPBX among others. This open source telephony application is written in C and it is build from the ground up.
OpenPBX - this is also another open source Private Branch Exchange system that is in software and can only be used by computers operating on the Linux operating system. This software is licensed under the GNU General Public License or GPL.

Article Source : http://www.look4articles.com/

Author Resource : This article touched the basics of the topic. I have 2 more resources related to the above. They are pbx
and ip pbxhow to unlock a blackberry. They are worth a read.