IP PBX is a Private Branch eXchange (PBX) system that uses Internet protocols for parts of its networking. Often, a PBX is applied to an enterprise, where IP connections are built into the company's intranet and where parts of that structure connect to the public switched telephone network for external communications.
IP PBX is also known as IP-Enabled PBX.
Techopedia explains IP PBX
PBX systems started to become very common in offices beginning in the 1990s. The idea was that, instead of adding manual telephone lines to an office building or other location, companies could add phone lines by creating digital switches within a digital structure that would include links to the traditional landline system outside of the office.
Over time, PBX systems became connected to Voice over Internet Protocol (VoIP) elements. New IP PBX systems often include helpful tools for video conferencing and combining voice and data services. Some of these include mobile telecommunication setups, as well as public telephone setups. Using strategies like Session Initiation Protocol (SIP) trunking, these telecom systems offer companies a broad infrastructure and sometimes help consolidate billing or streamline business processes.