According to a report on Intomobile, Skype for the Apple iPhone running on AT&T’s 3G network works really well, so it isn’t surprising AT&T has banned Skype from their 3G network.
As we have heard AT&T has reduced Skype for iPhone usage to WiFi Hotspots in the United States, something O2 has also done here in the UK, while T-Mobile Germany has banned the Skype iPhone application all together.
However there is a jailbreak application known as VoIPOver3G which allows jailbroken iPhones to use VoIP apps on 3G networks and once installed, all iPhone VoIP clients work on 3G, so to find out how follow the link.