Telephony is a telephone communication.
A telephone system with a private branch exchange which can be connected internally within an organization without using a public switched telephone network (PSTN/NTT network) is called an extension telephone. Large-scaled major companies apply PBX (Private Branch eXchange) to meet multi-function requirements like internal extension, ADSL, and private circuit connection. Smaller scaled companies apply key telephone system to serve an almost identical function as PBX.
Since digital systems became standard, factories, large firms and hospitals began to use PHS (Personal Handy-phone System) as their internal extension system.
It is now being integrated with computer by applying CTI (Computer Telephone Integration) at places like call centers, can show customer information on an operator’s terminal and update call-logs automatically with a data obtained by a numerical display. Recent days VoIP (Voice over Internet Protocol) is becoming popular as a means to make extension calls for free to meet TCO requirements. VoIP is a technology transmit one’s voice over an IP (Internet Protocol) network.
This is a system which converts voice data to an IP packet, and transmits it alongside other data over LAN. Internet phones and various other systems use this method.
More companies are now adopting PBX which uses an IP based switchboard due to the presence of high-speed broadband in the communication network.

• Assigning a number to each individual phone line

  It can take a significant amount of time to adjust PBX settings with changing office layouts or site relocations.

• Separate LAN and telephone lines.

  The cost of wiring is doubled.

• Assigning a number to each IP phone

  When changing layout, just plug in IP phone at the new location.

• Connecting phone line and PC to LAN cable or wireless LAN.

  Can be cost saving since the wiring is relatively short. In the case of wireless LAN, no wiring is required.