Since VoIP is a technology, it can be used for many applications: a telephone system; a private or public, local or long distance or international network; to save money on long distance or international calls; to connect multiple physical locations or to extend one's presence to virtual locations; to reduce operating costs or to enhance operating capabilities, etc.
While there is much in the technology that is new, there is very little it can do that hasn't been possible for quite some time using older technology (This is currently quaintly derided as "Legacy." We'll discuss that a little later).
Before you start reading about alternative IP-PBX scenarios, here are some working definitions of the relevant terms. A PBX or Pabx simply refers to a telephone system with an attendant (live or automated), a central system (real or virtual), and numbered extension telephones associated with individual users. A Pabx that uses VoIP is simply a Pabx that uses Internet Protocol (or IP) technology. Such a system is frequently called an IP-PBX.
It is important to note that these systems can be housed on your premises or they can be shared remotely by tens or hundreds or thousands of companies, in which case they are called "Hosted Systems"."They range in price from free Open Source software to proprietary hardware and software costing millions of dollars.
In fact, the possibility of a client offering VoIP service for resale is raised twice elsewhere in this site.
The next part of this section is Some IP-PBX Alternatives and Scenarios.