About 6 months ago I was working with a client that was interested
in using VOIP to reduce the telco costs of his hotels. He had several
locations spread between two states and he also managed an IT business
in India.

I created a series of example layouts in MS Visio so he told
understand the various methods of implementation. First let's look at
the simple layout:

This is a very simplified illustration of how an Asterisk server
that can receive inbound calls from great variety of callers.
Connections can be made by another office that has its own Asterisk
box, an office that is simply using a VOIP software phone or traveling
workers calling to the office through a conventional landline.

This is one of the easiest ways to setup a small office. In this
case the office is using their existing POTS phones and merely
connecting them into the Asterisk server. This will require you to have
an adapter installed. These come as either a PCI card or an external
switch. It accepts the standard phone line and converts to VOIP. The
PBX does not have to be connected to the internet to be functional in
this setup.

Here is a more detailed view of the office's extensions, but
additionally in this case the Asterisk box is hosted at a remote
location. The CoLo could be at a hosting facility or just at another
office. All of the extensions are VOIP phones connected through your
typical office LAN to the internet. No special network gear would be
required. I still included icons for how the office's PBX would then
connect to the landline users.

Here I did a simple mock-up of how the customer could later expand
the local system to save on overseas communications with his office in
India. He was mainly interested in using the PBX for their weekly
conference call, which Asterisk handles with ease.