Certain carriers allow you to make and receive calls on your iOS, watchOS, and Mac devices over Wi-Fi (and/or over cellular data on Wi-Fi + Cellular models of iPad when a Wi-Fi signal is weak or unavailable).
If you enable Wi-Fi Calling and connect to a Wi-Fi network from a Wi-Fi Calling enabled device, information about the country where your network connection is made, based on your IP address (or cellular network if applicable), may be sent to your carrier for billing and service availability purposes. If you do not want to share this information with your carrier, you may turn off Wi-Fi Calling on your iPhone and paired Apple Watch by opening Settings > Phone, and turning off Wi-Fi Calling (for iPhone) and Calls on Other Devices (for your other devices, including your paired Apple Watch). To turn off Wi-Fi Calling on an iPad or iPod touch, go to Settings > FaceTime, and turn off Calls from iPhone. To turn off Wi-Fi Calling on your Mac, open FaceTime, open Preferences, and turn off Calls from iPhone.
In order for your carrier to complete short code calls (calls to abbreviated numbers that provide access to public services) via Wi-Fi or cellular data, Apple must provide the city from which the call is placed to your carrier to help them properly route the call. If you do not want to share the city you are calling from with your carrier when making short code calls, you may turn off Location Services for Wi-Fi Calling on iOS and watchOS by opening Settings > Privacy > Location Services > System Services, and disabling Wi-Fi Calling. You can turn off Location Services for Wi-Fi Calling on macOS in the Privacy pane of System Preferences by clicking “Details” next to System Services and then deselecting Wi-Fi Calling. Please note that if you turn Location Services for Wi-Fi Calling off, your device will not be able to determine or send the city you are in to your carrier when you dial a short code number for a Wi-Fi call and as a result your carrier may not be able to properly route or connect your call.
Emergency calls on your iPhone are routed through cellular service when available. In the event that cellular service is not available and you have enabled Wi-Fi Calling, emergency calls may be made over Wi-Fi, and your device’s location information may be used for emergency calls to aid response efforts regardless of whether you enable Location Services. However, your carrier may choose to use your billing address or other address you provide to your carrier for the purpose of routing emergency services. You may update your address information for your carrier in Settings on your iOS devices and in FaceTime Preferences on your Mac. You should keep this information up to date. Failing to do so may prevent emergency services from being properly routed to your physical location. For location reliability reasons, it is preferable to make emergency calls over traditional cellular or fixed line services, and only use Wi-Fi Calling features for such calls as a last resort.