Wi-Fi 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 iCloudâconnected devices, you can also make and receive Wi-Fi calls on other devices. For details, see the Apple article Make a Call with Wi-Fi Calling.