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 or Skype, it uses the carrier's phone number, so caller ID functions normally. Call charges are the same as regular voice calls and are covered by unlimited calling plans. It also supports handover from Wi-Fi to mobile networks, so calls are less likely to drop when you move out of Wi-Fi range.
Availability in Japan varies by carrier. KDDI (au) offers "au Wi-Fi Calling," and Rakuten Mobile's Rakuten Link app also supports calls over Wi-Fi. NTT Docomo and SoftBank had not launched general Wi-Fi Calling services as of 2024. Overseas, major carriers such as AT&T, T-Mobile, and Vodafone offer it as a standard feature, putting Japan behind in adoption.
Setup is straightforward - simply enable Wi-Fi Calling in your phone's settings. On iPhone, go to Settings > Phone > Wi-Fi Calling. On Android, go to Settings > Network & Internet > Calls > Wi-Fi Calling. 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.