Now we have a look of what is a PBX or (Private branch exchange)……in a laymans term we can tell PBX is a telephone exchange in a home or in a office which can have all the features of a local telephone exchange….(it can be a hardware exchange or a software exchange)……

“A Private Branch eXchange (PBX) is a telephone exchange that serves a particular business or office, as opposed to one that a common carrier or telephone company operates for many businesses or for the general public. PBXs are also referred to as:”

• PABX – Private Automatic Branch eXchange
• EPABX – Electronic Private Automatic Branch Exchange

This is what wikipedia tells …..
as the normal telephone exchange do…we can make many extensions with a PBX can route our calls as the telephone exchange does…..for example 106 PBX means it will have 1 P&T conection and 6 extension….P&T(Post and Telegraphy) ..it is a connection that we get from our telephone exchange……and the extension means we have locally connected 6 extension phone with the PBX……..