said by shashi0807:vDialer is the best in its category with full integration of the Iphone contacts. No need to worry about calling the number of vonage phone. Just dial from the app and it will connect to your vonage phone.
Why not use the standard Vonage supplied iPhone app. I use it - I can make calls via wifi and 3g (jb only) using my contact list and have the minutes apply to my home Vonage account (unlimited worldwide). If you correctly link the app to your vonage number - this works GREAT.
I used almost 2000 minutes last month on my Vonage line and 15 minutes on my AT&T iphone account.
Note: To have the Vonage app run in the background and to have it work with 3g - you must have a jailbroken iPhone