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.