Private Automatic Branch Exchange (PABX)

The term “private automatic branch exchange (PABX)” is often used as a synonym for “telephone system”. Private automatic branch exchanges are switchboards which connect several devices, e.g. telephones or faxes and answering machines with each other and with the public telephone network. This allows a number of phones to be operated using a relatively low number of public telephone lines. Internal calls can be placed free of charge without using the public telephone network. Depending on the type of telephone system and the configuration, internal lines can be called directly from outside by using a direct dial in number, or indirectly through an operator.

Private automatic branch exchanges typically offer a number of features to make communicating significantly more convenient. Key components include toggle, conference calls and picking up calls. Toggle is used to switch back and forth between multiple calls. With call pick-up you can answer a call to an unstaffed phone from your own phone. With conference call several subscribers can be included in a call and everybody can talk with all parties to the call. Other typical features of private automatic branch exchanges are callback, call waiting, call transfer and call forwarding.

As telephone and communication technology advanced, the technology and functions of private automatic branch exchanges also changed significantly. In the early days, private automatic branch exchanges worked analogue and could connect to the public telephone network via several parallel lines. With digitisation and the introduction of ISDN, private automatic branch exchanges were often connected using so-called ISDN primary multiplex connections, which allowed 30 parallel connections with the public telephone network.

By switching telephony to Voice over IP communication, software-based functions of extensions can now be implemented through standard servers. These telephone systems signal via IP protocol with SIP to communicate with phones or the public telephone network. Voice over IP private automatic branch exchanges to be run completely cloud-based. In this case the actual business site doesn't require separate hardware for the private automatic branch exchange. An internet connection with sufficient bandwidth is all that's needed to use all the features of the cloud-based system.