Using the app, calls to China or to U.K. landlines are only $0.04 per minute, which presents a significant savings over using your cell phone to call directly. Pricing is also fairly competitive with providers such as Vonage.
The app can place regular calls in the U.S., with no WiFi needed, but does require WiFi in other countries in order to take advantage of the discounted calling rates. The new AT&T Call International app is a free download, and available immediately from the App Store (free, App Store Link).