Wifi calling can be useful in low reception areas. It's supported on Brown-paid iPhones on AT&T and Verizon and may also work on your personal iPhone as long as you have an iPhone 5c or later on a supported carrier. If your phone and plan support it, here's how to set it up.
On your phone, select Settings > Phone > Wi‐Fi Calling.
Enable the Wi-Fi Calling on This iPhone slider.
If your carrier supports Wi‐Fi Calling on iCloud‐connected devices, you can also make and receive Wi‐Fi calls on other devices. For details, see the Apple article Make a Call with Wifi Calling.