I have not found a specific listing of of the hardware and software that would be required to implement basic VoIP service for our small business. I would like a configuration diagram that details required servers, interfaces, routers, software, etc. Do you know where I might find this type of information?
The main reason you have not found this information, is that it varies greatly from vendor to vendor. Avaya has some great information on its site and a great product line. Nortel also has some good equipment information on its site.
In short, you will need a VoIP switch (also known as an IP/PBX). Some come with a gateway in the box. Here is how this works. When you place a call, if both endpoints are VoIP it will communicate that via your Internet line. The newer SIP equipment will do this via the same HTTP headers as a Web browser. If the call must go out to the regular POTS network, it will also handle that routing.
Next, you will need the phones. You have two options, regular IP phones or a softphone which works as software on a PC. If you have remote users, you will want them to have a VPN connection back into your switch so that their calls can be routed to VoIP, to maintain cost savings for all users.
Finally, you will need an Internet connection from your service provider. More than likely, this will include a router. You will want to make sure that your system supports E911 locations or that you do something to address that. Since IP phones can be anywhere, you will want to limit your liability and exposure by either trapping 911 calls to an operator locally or have some means to do so.
Of course, we are always here to help. Let me know if you need anything else.