Wi-Fi Calling is a feature that enables voice calls over a Wi-Fi network when cellular signal is weak. It serves as an alternative to VoLTE, particularly useful indoors and underground where radio waves have difficulty reaching. Technically, voice data is routed through Wi-Fi to the carrier's IMS (IP Multimedia Subsystem) via 3GPP's ePDG (evolved Packet Data Gateway) to reach the carrier's core network.
The greatest advantage of Wi-Fi Calling is that it uses your regular phone number for both incoming and outgoing calls, requiring no special setup on the other party's end. Unlike VoIP apps such as LINE, it uses the carrier's phone number, so caller ID functions normally. Charges follow whatever terms your carrier applies to ordinary voice calls, so check your own contract to see whether an unlimited calling plan covers them. Some setups also hand the call over from Wi-Fi to the mobile network, which makes a call less likely to drop as you move out of Wi-Fi range, though the exact behaviour differs between carriers and handsets.
Whether you can use Wi-Fi Calling depends on both your carrier and your handset supporting it. On a line whose carrier does not support it, the setting may not appear at all, or it may refuse to work once switched on. Apple's own guidance is that if you cannot turn Wi-Fi Calling on or use it, the first things to check are whether your carrier supports the feature and whether the device is running the latest software. Service names and availability differ from carrier to carrier and can change later, so the reliable answer for your own line comes from your carrier's own information.
Setup is done entirely on the handset. On iPhone, open Settings > Cellular > Wi-Fi Calling and switch it on; if you use more than one SIM, select the line you want to use for Wi-Fi Calling first. You may be asked to enter or confirm an address, which is used for emergency calls. On Android the option sits under Settings > Network & Internet > Calls, but its exact location and wording vary by handset and OS version. Note that emergency calls (110/119) made via Wi-Fi Calling may have reduced location accuracy, so using the mobile network is recommended for emergencies. See VoIP Basics for related technology.