Originally Posted by draver

WiFi calling uses a WiFi service in lieu of a cellular connection for phone calls. You can put your phone in Airplane Mode and still connect to cellular networks to place and receive calls. Both of your iPhones will allow WiFi calling. Your carrier must permit it, but T-Mobile has it available for all plans. It is simple to use and very useful in several ways. Most hotels and many restaurants provide free WiFi service to their patrons. If you are in a foreign country and wish to call to a US number, the call is free over WiFi. This also applies to WiFi service on cruise ships, trains, buses, etc. It typically is blocked by airlines who have the service however. Prior to your trip in your phone Settings-Cellular, you tap your voice number and then the WiFi Calling listing. This requires you have a valid Emergency Address in place, but here is where you also turn WiFi Calling On or Off. You may test it prior to departure by activating Airplane Mode with WiFi on, then making a call. Your Status Bar should read: "T-Mobile WiFi" or something similar next to your signal bars, along with the WiFi symbol. When you turn Off Airplane Mode, you should return to normal cellular service. T-Mobile requires that a WiFi call be made in the US prior to the service functioning overseas.

Dual Sim phones can also enlist a feature (IMS) that allows one line to use the other line as a pseudo WiFi service for simulated WiFi service and calling. This permits free US number calls while internationally roaming WITHOUT external WiFi service. Very useful and thrifty also. No need to be on a fixed WiFi signal while sightseeing.