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.