The installation instructions provided by Vonage (and even their web site) do not cover the situation for connecting the digital phone adapter (in my case the Motorola Voice Terminal VT1005V) to a switch servicing multiple static IPs. Here's a simplified schematic of my network:
ext IP 1
|----------VT1005V
|
DSL modem <---> switch |ext IP 2
|----------PC 1
|
|ext IP 3 |---PC 2 (internal net)
|----------Router---------|
|---PC 3 (internal net)
The nice thing about this configuration is that it avoids using the VT1005V as a router, a task for which it is probably not particularly well-suited. The real router does network address translation (NAT). (Note: The reason I needed the switch in the first place was because my router could handle only one external IP.)
The main task (a little more work than plugging in the cables) is to configure the VT1005V to use a valid external static IP. This is easy to do by temporarily configuring a PC to have IP address 192.168.102.2 and directly connecting the VT1005V via a cable from its PC port to the PC itself. Typing 192.168.102.1 in a browser brings up the desired configuration screen. There's plenty of documentation available on the web on how to use the configuration screen.
Feel free to send me an email if you have any questions.
Paul Takemura, paul@macriffs.com, February 3, 2006