PBX

A Private Branch Exchange is a physical device, normally located within the client’s premise. A PBX manages the internal call switching within an office and allows users, whose VOIP phones are connected to it, to share a limited number of public telephone lines.
A PBX is connected internally to the office telephones either directly or via a switch or router and externally to the internet via a dedicated ADSL line or the primary EOF connection. Once connected to the internet SIP trunks enable the PBX to connect to the public telephone network.
For example, a small office with 10 staff all have VOIP desk phones that are connected, using Ethernet cables, to a switch, the switch is then connected to the PBX and is responsible for sending data to and from the PBX to the correct telephone. In this scenario the 10 staff all share 7 external phone lines, meaning that no more than 7 of the team can call an external number at the same time and it’s the PBX’s job to dynamically allocate external phone lines and connect internal calls over the local network.