Android users have been able to use Skype and other popular clients to make Voice-Over IP calls for a while now, but AT&T customers now have a more official option. AT&T Call International allows for native VOIP calls to international numbers. Prices for international calls start at 4 cents a minute, and AT&T users can download the app from the Android Market now. Blackberry and iPhone versions are also available.
Unfortunately, the app doesn't really take advantage of the possibilities of VOIP, like automatically connecting to other AT&T Call International users for free calling. In fact, there really isn't anything to recommend the app over Skype or Google Voice. There's no additional charge for calling numbers in the US or Puerto Rico, but if you're traveling and happen to be calling from a non-US location, you can't use 3G or 4G networks - WiFi is the only option.
The app makes use of a custom phone dialer, but ties into Android's Contacts application so retrieval of numbers is easy. Naturally only AT&T customers can see the app in the market. In an off move billing will be applied directly to customers credit card, not their AT&T bill. You can expect the AT&T Call International to be pre-installed on Android phones in the future.