If you are using the latest Apple iPhone 6 or iPhone 6 Plus, then you should be aware of the Wi-Fi calling function that comes with the devices. However, this golden feature on the sixth-gen iPhones is only available on T-Mobile and Sprint.
There is a lot of confusion on how Wi-Fi calling works. Some claim that they made calls via Wi-Fi connection but it still sucks away the air time on their carrier price plan. Well, there is a solution to this and it is through applying Airplane Mode prior to making a call.
This will actually prevent the iPhone 6 from switching to the carrier network whenever the W-Fi signal gets killed. On the other hand, the Airplane Mode will actually result in the call getting dropped whenever the Wi-Fi signal weakens or got disrupted.
Regardless, the above should give you a rough idea on how Wi-Fi calling works and this will allow you to find the best method in using the feature.