Wi-Fi Calling is a fairly new feature from Verizon, ATT, and other cell phone providers. It lets you talk and text over a Wi-Fi connection, even when cellular coverage is limited or unavailable.
It is available on iPhone 5c, 5s, SE, 6, 6 Plus, 6s, and 6s Plus models. And it should be available on any new iPhone models that come out.
Many of us who live (or have lived) in places without good cellular coverage and gone out and bought devices from the provider that you attach to a router in the hopes of making and receiving calls. Verizon's Network Extender worked well. ATT's MicroCell, not so much.
Now we don't need those things. If you connect your iPhone to Wi-Fi, you can make and receive calls, and text. No land line required. No pricey networking gear either.
One of my clients was giving out her personal land line number to her business contacts so she could talk to them at home. It was unprofessional and often calls came in on the wrong number. She was thrilled to find out about Wi-Fi Calling.
To setup Wi-Fi Calling, go to Settings > Phone. Then turn on Wi-Fi calling. You will be prompted to enter an emergency address. This is because using W-Fi Calling for a 911 emergency call does not automatically send the location information, like it does with a land line So if you are able to place a 911 call, but not able to speak, the system needs a default address to use. When setting up Wi-Fi Calling you must enter in the address for the system to use in such cases.
In some cases, turning on Wi-Fi calling doesn't work. Either call your cell provider on a land line or call 611 on your iPhone. They will sort out the problem and should get it working. Sometimes just restarting you phone will make it possible to turn on Wi-Fi Calling.