Hi Steven,
Apple have specifically chosen an internet protocol for Voice over the internet to get iChat to connect to other people.
This Protocol is SIP (Session Initiation Protocol). It is alos used by VoIP services like Vonage.
Unfortunately some modems come in a default state Aware of SIP and look for a phone to connect to rather than any application on a computer.
The first thing to try is to move the Vonage adaptor to the same level as the computer.
Normal set up
Modem <-> Adaptor <-> Computer
You need
Modem <-> Router <-> Computer and adpator
If the modem is SIP Aware then you may need to UnBind SIP from port 5060 to allow it to go to any device.
Info on SIP
http://www.ralphjohnsuk.dsl.pipex.com/page4.html#_text and instructions on how to
telnet into your modem and instruct the unbinding.
Ralph