If you want to risk the ire of Apple and AT&T, you can use the Skype iPhone application over a 3G connection. Usage requires "jailbreaking" the iPhone and installing another client app.
In the U.S. Skype for iPhone will only initiate calls over WiFi hotspots, and will not allow users to direct a VoIP call over AT&T's 3G data network. Other carriers, such as T-Mobile, won't even allow Skype for iPhone near their networks.
Into Mobile walks through the steps necessary to first unlock the iPhone 3G, then load up a jailbreak application called VoIPOver3G to enable jailbroken phones to use VoIP apps over a 3G network. The VoipOver3G app installs and runs transparently and once done will allow all VoIP apps - not just Skype for iPhone - to work over AT&T's 3G network.
The software workaround is likely to cause more heartburn for AT&T and other mobile carriers placing limitations on applications over data networks.
For more:
- Into Mobile walks into the gray area of jailbreaking for Skype. Post [1].
Related articles
Advocacy group cries foul over iPhone Skype app ... [2]
Skype iPhone restrictions stir up lobbying to Europe, FCC - FierceVoIP [3]
Skype for iPhone: One week, 2 million downloads - FierceVoIP [4]
Links:
[1] http://www.intomobile.com/2009/04/30/skype-for-iphone-works-well-over-att-3g.html
[2] http://www.fiercebroadbandwireless.com/story/t-block-voip-app-iphone-advocacy-group-cries-foul/2009-04-04
[3] http://www.fiercevoip.com/story/skype-iphone-restrictions-stir-lobbying-europe-fcc/2009-04-04
[4] http://www.fiercevoip.com/story/skype-iphone-one-week-2-million-downloads/2009-04-08