A Private Branch Exchange (PBX) is a telephone system within a business that switches calls between users on local lines, while allowing all users to share a certain number of external phone lines. The PBX allows telephone users to set up circuit-switched voice calls between other users in the same business or to set up calls across the Public Switched Telephone Network. Users calling into the business dial a single number, while the PBX routes the call to the appropriate extension. Internal users (operators) have a number of outgoing lines for making calls over the telephone network. The main purpose of a PBX is to save the cost of requiring a line for each user to the telephone company's central office. The PBX is owned and operated by the business rather than the telephone company. The PBX is a stored-program, common-controlled type of device. As a telephone system, the PBX acts as a resource-sharing system that provides the ability to access a dial tone and outside trunks to the end user. This stored-program controlled system is an all-digital design structure. Private Branch Exchanges originally used analog technologies, but today digital systems are predominately used (Williams). .
Private Branch Exchanges can be used in many applications. Some applications include, but are not limited to the following; small office, individual business departments, conventional home use and school/institute applications (Williams).
The analog system uses analog type components to accommodate the call set-up and tear-down for the entire system. A voice call is introduced into the system in the same fashion that a business or residential user's input is introduced to the telephone company network. When making a call, the telephone handset is picked up from the switch hook and an Input Output (I/O) request signal is sent to the main frame of the PBX. Once the signal is sent, the system immediately returns a dial tone.