An IP enabled PBX can route the telephone calls over an IP WAN or over the the existing telephony network. It usually contains IP station and IP trunk cards. IP station cards allow IP based phones to appear as circuit switched phones and IP trunk cards allow an IP WAN to appear as a telephony trunk to the PBX.
In a telephony enabled LAN system, a communication server software plays the role of the traditional PBX and provides the call setup, address translation, routing and supervision functions to IP phones. An application server on the network usually provides enhanced business features.
Instead of Or in addition to on-premises PBX or Comm Servers, for small businesses, it may make sense to make of use Centrex services provided by a Carrier. The trade-off is between sacrificing some degree of control for added degree of convenience.
All these deployments use Gateways to bridge calls between the PSTN and IP networks. Depending on the amount of traffic and the local telephony interfaces, gateways will connect to the PSTN network via T1/E1 trunks, analog lines, etc. on one hand and to the IP networks with suitable bandwidth on the other.
User terminals (i.e. devices to make calls) for IP PBXs can be:
IP phone, that performs the voice to IP packet and IP packet to voice conversion
Soft phone which is a software based phone that exists on a PC and uses a microphone and speakers or a headset
Existing circuit switched phone with an adapter that allows it to connect to the IP network
PDA and/or cell phones with VoIP client software etc.
IP terminals usually implement H.323 or SIP or both protocols to talk to the PBX or to the communication server as the case may be.
IP PBXs have many advantages over standard PBX systems. They can be summarized as lower costs and newer easy to manage applications. Pure IP PBXs would be a good solution for new locations since only one network would need to be installed and maintained, and adding IP trunk capabilities to an existing PBX would be a good solution for companies that have branch offices to save on long distance calls between their locations.
Some issues with IP PBXs are QoS (Quality Of Service) and Reliability. QoS relates to jitter and lost packets when the network is busy with other non voice traffic over the corporate LAN or WAN or more likely the public network. New networking protocols (such as MPLS, DiffServ etc.) have been developed to address QoS issues and give appropriate priority to Voice traffic. Existing PBXs are designed and built to provide nearly 100% up time. Since IP PBXs are based on PCs and software, it is often difficult for them to achieve the same reliability as standard PBXs.
The resources listed above shed more light on the subject of IP PBXs.