This is pretty neat. I wish Vonage had a web API, but they do not. However, they do have a HTTP GET and POST based API for initiating a phone call from your Vonage phone.

Simply create a custom URL for your account:

https://secure.click2callu.com/tpcc/makecall?username=username&password=password&fromnumber=fromNumber&tonumber=toNumberWhere:
username
The name that you use to log in to the Vonage dashboard
password
The password that you use to log in to the Vonage dashboard
fromNumber
Your Vonage phone number, this must match one of the numbers returned from the getnumbers request above
toNumber
The phone number you want to dial

When executed, a call will first be placed to your Vonage number. The system will dial the outbound number after the Vonage phone is answered. The Vonage customer will hear ringing as if the number was dialed from their phone and the two parties will be connected.

The neat thing about this is you could hack up some ASP.NET code, place on your web page, and allow people to call you for free. Basically they would go to your web page and initiate a call from your phone to their's. When you answer your Vonage phone, the account will then dial their number.

Here are some handy shortcuts. The will allow you to type dial 18005551212 to dial a number. Replace the items in red with your settings.

SlickRun

magicword=dial
filename=iexplore.exe
parameters=https://secure.click2callu.com/tpcc/makecall?username=username&password=password&fromnumber=fromNumber&tonumber=$W$

MSN Desktop Search

@dial,https://secure.click2callu.com/tpcc/makecall?username=username&password=password&fromnumber=fromNumber&tonumber=$W