Wi-Fi calling explained: how it works and how it differs from calling apps
Wi-Fi calling lets your phone make and receive ordinary calls and texts, on your normal number, through a Wi-Fi connection instead of the mobile mast. To the person you call, nothing changes. To you, it can mean signal in a basement flat, a stone farmhouse or a hotel room abroad.
Wi-Fi calling is not the same as a calling app
Both send your voice over the internet, so they are easy to confuse. The difference is who runs the call:
| Carrier Wi-Fi calling | Calling app (WhatsApp, Signal, FaceTime…) | |
|---|---|---|
| Number used | Your mobile number | An app account (often linked to your number, but not the phone line) |
| Who you can call | Any phone number | Usually only other users of the same app |
| Where it appears | Built into the phone dialler | Inside the app |
| Billing | Your operator's tariff (see below) | Free beyond data, unless you buy credit for calls to numbers |
| Emergency calls | Supported on many networks | Generally not possible |
| Needs operator support | Yes, and a compatible phone | No |
How it works under the hood
Modern mobile networks handle voice as VoIP inside an operator system called the IMS (IP Multimedia Subsystem). On 4G that service is VoLTE; on 5G standalone networks it is VoNR. Wi-Fi calling, also called VoWiFi, simply gives your phone a second way into the same IMS.
Because a café or home Wi-Fi network is not trusted by the operator, the phone first builds an encrypted IPsec tunnel across the internet to a gateway run by the operator, called the ePDG (evolved Packet Data Gateway). The 3GPP specifications describe this as "untrusted non-3GPP access", and the GSMA IMS profile for Wi-Fi access (document IR.51) set out what phones and networks must support so that services from different vendors work together. Once the tunnel is up, the phone registers with the IMS exactly as it would over 4G, and calls are set up with SIP like any other VoLTE call.
On networks that support it, a call can be handed over between Wi-Fi and VoLTE as you walk out of range, although how smoothly this happens varies by operator and handset.
What you need
- An operator that offers it, and in some cases a plan that includes it. Prepaid and budget brands do not always enable it.
- A compatible phone. Support is not only a question of the model: phones bought from one operator, or imported from another country, may lack the operator-specific settings that enable Wi-Fi calling on your network. Your operator's website usually lists supported models; our guide to device support explains why.
- Wi-Fi calling switched on. On iPhone it is under Settings › Phone › Wi-Fi Calling (Apple's instructions); on Android the setting is usually in the network or call settings, depending on the manufacturer.
- An emergency address, in countries that require one. In the United States, for example, carriers ask you to register a street address when you enable the feature, so that emergency services can be sent to you if your location cannot be determined otherwise.
What it costs
Wi-Fi calling does not use your mobile data allowance, because the traffic goes over Wi-Fi. Calls are normally charged according to your plan, as if you were on the mobile network at home. The interesting case is abroad:
- Many operators bill Wi-Fi calls made from abroad to your home country as domestic calls, which can be a real saving outside free-roaming zones.
- Calls from abroad to numbers in the country you are visiting, or to a third country, may be charged as international calls.
- Some operators disable Wi-Fi calling in certain countries, or only allow it on their home network.
Because these rules are commercial choices, the only reliable source is your operator's tariff page. Inside the EU, roaming on mobile networks is usually included anyway; see EU roaming rules.
Wi-Fi calling and call quality
A Wi-Fi call is only as good as the Wi-Fi. A strong signal on a congested or slow network can sound worse than weak 4G. The same factors that affect any VoIP call apply: delay, jitter and packet loss. Hotel and public networks that block VPN-style traffic can also prevent the IPsec tunnel from forming, in which case the phone quietly falls back to the mobile network. If calls keep dropping, our call quality guide lists the usual culprits, including router features that interfere with VoIP.
Privacy and security
The tunnel between your phone and the operator is encrypted, so other users of a public Wi-Fi network cannot listen in. That is not end-to-end encryption: once inside the operator's network, a Wi-Fi call is handled like any other phone call, with the same records and lawful-interception obligations. If you need a call that only the two participants can decrypt, an end-to-end encrypted app is the tool; see what calling apps actually encrypt.
When to use which
- Weak indoor signal at home: Wi-Fi calling, so people can still reach your number.
- Travelling outside free-roaming zones: Wi-Fi calling for calls home if your operator charges them as domestic; app calls for anyone who has the same app.
- Calling family abroad who use the same app: an app call, free and often higher quality.
- Emergencies: the phone dialler, never an app. See emergency calls over VoIP and Wi-Fi.