Face Time Audio Calls allows you to make free international phone calls from your iOS7. You can utilize this feature from iPhone 4 and the latest models that capable to upgrade to iOS7.
FaceTime is a nice add-on from Apple which allows to make free calls to another iOS user without utilizing cell phone service provider minutes. In addition to this, there are iPhone Apps to Make Free Calls to US Landline and Mobile Number.
The only downside of this call is it may use your 3G or 4G data unless you are not able to connect through WiFi while making the Face Time Audio Calls.
Fortunately, iOS 7 brings up the new feature that will help you to choose Audio or Video Face Time Calls per user choice anywhere in World. When you open your contact list you can see audio and video buttons near to FaceTime feature.
The only requirement for this call is that both device should be in iOS platform which supports FaceTime. You are free to call unlimited calls using this feature to anywhere in the world.
If you are calling to number that is not supporting FaceTime or not using iOS device, don’t worry, you will end up error message like this. Tap on Audio Call button near to FaceTime. You will get an error message if that phone number is not a iOS number or not supporting FaceTime Call.
This is a nice feature to make audio calls to an iOS number anywhere in the word as absolutely free. It is neither going to charge you International rates for the calls nor utilize excess data for your FaceTime video transferring that was offered by iOS6.
Please see this post How to Watch and Limit Data Usage in iOS7 if you are worried about your excessive data usage in iOS 7.