There's a somewhat hidden feature in iOS called WiFi Calling, which lets you use WiFi to make and receive calls and text messages instead of relying on your carrier's cellular network.
WiFi Calling is great when your iPhone is out of reach of your carrier's cell signal at home or at your workplace where you have access to WiFi, and it'll even work when you're connected to public hotspots, like at coffee shops.The feature isn't turned on by default, so you have to go into your iPhone's settings to do so: Go to Settings > Phone > WiFi Calling.
Then you'll be asked to enter your address to register WiFi Calling with your carrier. This is mainly so emergency services could come to that address in case they couldn't locate you with a cell signal.
After a couple of minutes, WiFi Calling will be enabled, and you can start using a WiFi connection to make calls or send and receive text messages.
With WiFi Calling, you can even set up other iCloud-connected devices — like an iPad, iPod, or Mac — to make calls and send texts over WiFi. They don't need to be connected to the same network as your iPhone.
So far, AT&T, Verizon, Sprint, T-Mobile, MetroPCS, and Vodafone US support WiFi Calling in the US. Apple has a full list of international carriers that support WiFi Calling. Note that certain carriers don't offer the feature for post-paid plans, like AT&T's GoPhone.
Source: businessinsider.com