PBX Overview:

PBXs operate as a connection between a private organization -- usually a business -- and the public switched telephone network (PSTN). Because they incorporate telephones, fax machines, modems, and more, the general term "extension" is used to refer to any end point on the branch. The PBX handles calls between these extensions as well as connections to the PSTN via trunk lines.

PBXs are differentiated from "key systems" in that users of key systems manually select their own outgoing lines, while PBXs select the outgoing line automatically.

Key Systems Overview:

A key system or key telephone system is a multiline telephone system typically used in small office environments.

Key systems are noted for their expandability and having individual line selection buttons for each connected phone line, however some features of a private branch exchange such as dialable intercoms may also commonly be present.

Key systems can be built using three principal architectures:

• Electromechanical shared-control
• Electronic shared-control
• Independent keysets
  
  

VoIP Overview:

Voice over Internet Protocol , also called VoIP, IP Telephony, Internet telephony, Broadband telephony, Broadband Phone and Voice over Broadband is the routing of voice conversations over the Internet or through any other IP-based network.