Create Web Application to call a phone using VOIP

I want to create an application that can reside on my website, and I can dial any number in the world to call, over VOIP. It can be something similar to Skype, or GTalk or iCall etc. Please propose your solution if you have it available today, and the fixed price for it. Or if you can develop it, then quote the price, methodology, and your proposed idea and approach. It should be implementable in a Windows Server environment. Budget is $500.