As it turns out any Vonage line can be used as a SIP line. All you have to do is find out the domain and your encrypted password. Doing so is actually quite the simple process. Enter the following into your browser, making sure to replace the parts in bold with your account information.

https://im-config.vonagenetworks.net/config?&login=USERNAME&password=PASSWORD&type=1003

Update 09/07/2009: Unfortunately our friends at Vonage have taken down the above link. As of now I know of no other way to obtain your encrypted password, I will try to find an alternate method and when I do I will put up a new post.

You’ll get a page with a bunch of information on it, the part you want to pay attention to is at the top:

[SIP 2.0]
ProxyUserName=username
ProxyUserPassword=FAB16g9XNa
ProxyDomain=a.vonim.com
ProxyPort=10000

This contains all the information you need to register your account on Fring or SIAX. In the past I used SIAX however they decided one day to make it so you had to purchase the software, which defeats the purpose of Jailbreaking your iPhone (in my opinion). So now I use Fring, which can be downloaded for free in the App Store. Configuring SIP in Fring was a little less than straight-forward for me, so here are the steps to set it up:

Create an account or login to your existing account
Go to More then Add-ons
Select SIP, then Other

Fill out the fields as follows:
User ID = ProxyUserName
Password = ProxyUserPassword
Proxy = ProxyDomain:ProxyPort

Now save those settings and try making a SIP call. If you don’t see the SIP option lit up on the Dialer pad then you may just need to wait a few seconds for it to connect to Vonage