"A PBX or “Private Branch Exchange” is like
a miniature phone company/switch in your own office/house. The PBX
is responsible for making sure calls are routed properly.
Situation: when Joe wants to dial Suzy in the office down the hall
or downstairs in the TV room, he just picks up the phone in his
room/office and dials a local extension just a few digits long. If
he wants to call someone across town or around the world, he dials
an “outside line” prefix (usually a 9) plus the desired
number and the PBX routes his call to the PSTN (Public Switched
Telephone Network) just like any other dialed call. Also, if Joe
has four teenaged kids, instead of answering their calls and acting
as the house receptionist, he can just setup a menu that allows
callers to dial the person they want and then needs not be
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