VoIP stands for Voice over Internet Protocol, is a protocol used to transfer voice conversation over internet. PBX stands for Private Branch Exchange. A PBX is a switching system that manages the calls between local users. So we can define VoIP PBX as a switching system that connects all the voIP calls between local users. A PBX also provides lines that are shared for connecting to external phones.
A VoIP PBX also provides features like taking up a call from external network and redirects it to correct extensions or destinations, or send them to automatic answering service. VoIP PBX system consists of a phone line and server system that performs the analysis of the destination machine. The VoIP PBX server is capable of performing high rated tasks like receiving, redirecting, and answering incoming voice calls. 
The server can distinguish the calls and make use of respective handles for that. It is also highly programmable and many advanced functions like voice menu systems, Single click-to-call, call logging and tracking, automatic call conferencing etc, can be used.
Use of VoIP PBX system helps company to reduce the expense, by having a single network for both voice and data transfer. Further the VoIP calls do not cause any call charge if it’s between two systems, so its cost effective method to communicate in a network. And it also provides efficient way for call management. With call tracking and logging system in VoIP PBX, the server can log any incoming and outgoing details which will help to analyze any conversation made. It also provides way to block incoming calls.
A VoIP PBX is also known as an IP PBX , because VoIP systems run over IP (Internet Protocol).
