Along with the launch of iOS 7, Apple introduced FaceTime audio calls. This allows you to make a call over the internet instead of your carrier's voice network.
This is great when you're in a place with bad cell reception, but good Wi-Fi, but this feature is especially great when communicating with someone overseas. It eliminates all international voice charges, as there is no cost for this call, the quality is far superior to your carrier's voice network, and no special apps are needed. Note that if you or the person you're calling is on their cellular data connection, it will use data from your cellular data package.
To make a FaceTime audio call, find someone's contact and tap the Phone Handset icon beside "FaceTime". If you're using this a lot, you can even add a FaceTime audio entry to your Favorites list.