Voice over Internet Protocol (Voice Over Internet Protocol (VOIP)) , that it is also called IP Telephone, is rapidly becoming a familiar term and technology that put governmental and scientific organizations and institutions under the onslaught. Voice over Internet Protocol technology is designed for replacing TDM (Time Division Multiplexer) and existing networks with data networks based on Internet Protocol. In these networks , digitalized audio in the form of Internet Protocol data packets will be carried through LAN (Local Area Network (LAN)) and / or city (Metropolitan Aria Network (MAN)). The basic steps for setting up an internet phone calls involves the converting analog voice signal to digital format and compression and translation of signales into Internet protocol packets for transmiting over the Internet, which is inverse in recipient side. Installation of Voice over Internet Protocol networks need new tools.

IP-PBX is a phone system which is designed to deliver either voice or video via a data network, cooperated with Public Switched Telephone Network. VoIP (Voice over Internet Protocol) gateways can be combined with traditional PBX functionality enabling businesses to use their own managed intranet to help reduce expenses.

Similar to a traditional PBX, IP-PBX belongs to an organization. With a standard PBX, distinct networks are required for voice and data communications. One of the primary advantages of an IP-PBX is the fact that this system works with voice and data networks simultaneously. This implies that internet accessibility is possible as good as Voice over Internet Protocol and telephony communications. Hereby, flexibility is increased as organization improves.

An IP-PBX can exist either as a hardware object or as a virtual software system. It controls voice communications via an organization and acts as a gateway to voice and data networks. And IP-PBX enables us to establish connections through a network, instead of using standard telephone infrastructure. Telephones can be connected to IP-PBX via a network and calls may be traced By IP-PBX rather than Public Switched Telephone Network. A common IP-PBX can switch calls between VoIP users and a traditional telephone subscriber, or act as the way a standard PBX performs between two traditional telephone users. According to statistics, IP protocol-based systems are being developed increasingly.

An IP PBX or phone system is consisted one or more SIP (Session Initiation Protocol) phone, IP PBX server and a VOIP gateway, which can be connected to the public telephone switching network lines. the operation of IP PBX server is like proxy servers : SIP clients services are hardware or software phones that are registered in the IP PBX server , and when they want to make a call they request to establish a connection to IP PBX . IP PBX has a directory of all phones and users SIP addressing, therefore it is able to connect to an internal call or provide an external call via a VOIP gateway or a VOIP service provider.