Here's a simple tutorial on how to make Skype and other VoIP calls using 3G on a jailbroken iPhone:
Yet another reason why I am glad to have a jailbroken phone. Yup, this one is limited to those who are jailbroken, or willing to jailbreak. Anyway, it is possible, even easy to make VoIP calls on the iPhone using a cellular connection, which in my case is AT&T's 3G network.Source: blog.makezine.com