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Private Branch Exchange 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). PBX