What is WiFi Calling? How it works and how to use it, explained simply

What is WiFi Calling?

WiFi Calling is a way to make and receive calls on your own mobile number over a Wi-Fi network instead of the signal from a cell tower. The person you call sees your usual number, and you dial from the same Phone app you always use.

In technical documents it is called VoWiFi (Voice over Wi-Fi), and each carrier brands it a little differently: AIS calls it AIS Wi-Fi Calling or AIS 4G VoWiFi, and NT calls it WiFi Calling (VoWiFi). They are the same technology. The difference from a LINE or WhatsApp call is that WiFi Calling uses your real phone number and runs through your mobile carrier's system, not an app company's servers.

• Call and receive calls on your usual number, to and from any phone, including landlines and people who have no messaging apps.
• Send and receive SMS through the same system, if your carrier has enabled SMS over IP.
• Works where mobile signal is weak, such as basements, high-rise condos or rooms with thick walls, as long as the Wi-Fi is good.
• Can be used abroad, if your carrier allows it, without attaching to a foreign roaming network.

Short definition of WiFi Calling in the glossary

How does WiFi Calling work?

The core of WiFi Calling is IMS (IP Multimedia Subsystem), the carrier system that handles all IP-based calls. VoLTE calls over 4G and VoNR calls over 5G go into the same IMS. The only difference is the way in: VoLTE enters through a 4G tower, while WiFi Calling enters through the internet over Wi-Fi, a network the carrier does not own. Because that path is untrusted, the carrier puts a security gateway in front of its core, called the ePDG (Evolved Packet Data Gateway).

1. 11. The phone joins Wi-FiWith WiFi Calling switched on and a Wi-Fi network that reaches the internet, the phone looks up the ePDG of the carrier that issued the SIM.
2. 22. The SIM proves who you areThe phone and the ePDG negotiate using IKEv2 and authenticate with EAP-AKA, which uses the secret keys stored on the SIM. This is why WiFi Calling always needs the carrier's SIM or eSIM in the phone, even when no mobile signal is used at all.
3. 33. An IPsec tunnel is builtOnce authenticated, the phone and the ePDG set up an encrypted IPsec tunnel, and everything about the call travels inside it, so the Wi-Fi owner cannot see the conversation. The connection uses UDP ports 500 and 4500; a network that blocks those ports stops WiFi Calling from connecting.
4. 44. The phone registers with IMSThrough the tunnel the phone registers with IMS using SIP, just as it does on 4G. This is the moment the Wi-Fi Calling or VoWiFi indicator appears on screen, meaning the phone is ready to call.
5. 55. Calls and texts flowYour voice is compressed with the same codecs VoLTE uses, such as AMR-WB or EVS, sent through the tunnel to IMS, and the carrier connects the call onward as normal. Incoming calls to your number come back the same way.

The rules for all of this come from 3GPP specifications for untrusted non-3GPP access to the mobile core, and from the GSMA profile IR.51, which sets the minimum feature set for voice, video and SMS over Wi-Fi. Phones and carriers that follow the same profile can work together.

How is WiFi Calling different from VoLTE and LINE calls?

All three use internet protocols, but they differ in the path the call takes and the number it uses. The table sums up the points people mix up most often.

WiFi Calling vs VoLTE in detail

Does the call drop if you walk out of Wi-Fi range?

Not necessarily. Because WiFi Calling and VoLTE use the same IMS, a carrier that supports handover can move a live call from Wi-Fi to VoLTE on 4G, or back again. During the move the phone carries the call's IP address across to the new path, so IMS still sees the same call.

In practice there are conditions: VoLTE must also be on, the carrier must have handover enabled, and there must be 4G signal from your carrier where you are. Abroad with Airplane Mode on, there is no 4G to hand over to, so the call ends when Wi-Fi drops. For an important call while travelling, stay where the Wi-Fi is strong and steady.

Most Android phones let you pick a mode such as Wi-Fi Preferred or Cellular Preferred. True-dtac's help page tells Android users to choose Wi-Fi Preferred. iPhone has no such option and decides on its own.

What do you need to use WiFi Calling?

WiFi Calling is not a switch that works in every case. All of the conditions below must be met; if one is missing, the menu may not appear, or it appears but calls do not go through.

• A supported phone: Apple says you need an iPhone 5c or later on a supported carrier. On Android it depends on the model and its firmware, not only the hardware.
• An active SIM or eSIM from a carrier that has enabled WiFi Calling for that number.
• The carrier must support your phone model too. Carriers switch the feature on through carrier settings (carrier config) only for models they have tested.
• Some grey-import phones, especially Samsung units with a firmware region (CSC) from another country, may have no WiFi Calling menu at all, even though the hardware could do it.
• Wi-Fi that really reaches the internet and does not block UDP 500 and 4500. Some hotel or public Wi-Fi that needs a login page, or that blocks VPN traffic, will stop WiFi Calling from registering.
• For True and dtac: WiFi Calling must be switched on while you are still in Thailand and left on before you travel, as True-dtac's help page states.

Check which phones support WiFi Calling

Which Thai carriers offer WiFi Calling?

Apple's Asia-Pacific carrier features page lists AIS, dtac and True in Thailand as supporting both Wi-Fi Calling and VoLTE on iPhone. NT Mobile also runs its own WiFi Calling (VoWiFi) service. How you switch it on and the conditions differ by carrier:

On cost, every carrier checked says WiFi Calling is charged against your package like a normal call, so it is not automatically free. The real price depends on your own package; for exact figures, check your carrier's app or website.

AIS WiFi Calling: setup and conditions

How to turn on WiFi Calling on iPhone and Android

Menu names vary a little by model, OS version and carrier, but the main locations are these:

1. 1iPhoneGo to Settings > Cellular (Mobile Data or Mobile Service in some regions) > Wi-Fi Calling and turn on Wi-Fi Calling on This iPhone. With two lines, tap the number you want first. You may be asked to enter or confirm an address for emergency services, per Apple's guide. On newer iOS versions the menu can move; if you cannot find it, type Wi-Fi Calling into the Settings search bar.
2. 2Samsung GalaxyGo to Settings > Connections > Wi-Fi Calling and turn on the switch, per Samsung's guide. On many models you can also reach it from the Phone app > three-dot menu > Settings, and you can add a Wi-Fi Calling tile to the Quick settings panel.
3. 3Google Pixel and other Android phonesOn Pixel it is usually Settings > Network & internet > SIMs > choose the SIM > Wi-Fi calling. On Xiaomi, OPPO, vivo and realme it usually sits under the SIM cards and mobile networks menu > choose the SIM > Wi-Fi Calling or Make calls using Wi-Fi. The fastest way is to type Wi-Fi into the Settings search bar.
4. 4Activate with the carrier if neededAIS users on Android dial *399*1#, and NT users dial *834#. True and dtac users just use the phone's switch. If it still does not appear, call your carrier, since some numbers may not have the feature enabled yet.
5. 5Check that it really worksConnect to Wi-Fi and look for Wi-Fi Calling, VoWiFi or a phone-with-Wi-Fi icon in the status bar. On iPhone, open Control Center and you will see Wi-Fi next to the carrier name. On many Android phones, dialling *#*#4636#*#* opens a menu that shows whether IMS is registered. Then make a short test call to someone you know.

Step-by-step setup for every brand

Can you use WiFi Calling abroad?

Yes, if your carrier allows it and you switched it on before leaving Thailand. This is why many Thai travellers use it: you can take calls on your Thai number and call home over hotel Wi-Fi without voice roaming. AIS and True-dtac both give the same routine: turn on Airplane Mode first, then turn Wi-Fi back on.

WiFi Calling runs on your own Thai number, so your AIS, True or dtac SIM must stay in the phone. A travel eSIM such as the ones esimonline sells is data-only, with no phone number and no WiFi Calling of its own. That is why many people run a dual-SIM phone: the Thai SIM for the number and OTP texts, the travel eSIM for internet.

The next question is whether, with no Wi-Fi around, the Thai SIM's WiFi Calling can ride on the travel eSIM's mobile data. Some brands have a feature that lets one line call over the other SIM's data, but behaviour differs by model, OS and carrier, and we found no official Thai carrier page confirming it works abroad. The dependable routes are real Wi-Fi, or, if you carry a second device, sharing a hotspot from the travel eSIM on that device and joining it as Wi-Fi from the phone that holds your Thai SIM.

Guide to using WiFi Calling abroad

See data eSIMs for travel

What are the upsides and limits of WiFi Calling, and does it use much data?

On data, WiFi Calling uses the same voice codecs as VoLTE, which are light. Several guides estimate a voice call at roughly 0.5 to 1 MB per minute including tunnel overhead; the real figure depends on the codec and network. On unlimited Wi-Fi this hardly matters.

On emergency calls, Apple says iPhone uses the mobile network for emergency calls when it is available and might use WiFi Calling when it is not. Abroad, the local emergency number is better dialled over a local mobile network.

• Menu missing: check the model is supported, update the OS and carrier settings, then ask the carrier whether the number is enabled.
• Switched on but no indicator: try another Wi-Fi network, turn off any VPN, or restart the phone. On Wi-Fi with a login page, finish the login first.
• Forgot to set it up before travelling (True-dtac): True's help page suggests connecting to Wi-Fi and dialling ##002# to cancel call forwarding to voicemail.

WiFi Calling not working? Fixes

Read next: every article in the WiFi Calling series

This page is the overview. For detail on one topic, pick from the list below.

Does WiFi Calling cost money?

Does WiFi Calling use data, and how much?

WiFi Calling on iPhone

WiFi Calling on Android

WiFi Calling on Samsung

WiFi Calling option not showing

Can you get SMS over WiFi Calling?

True WiFi Calling

dtac WiFi Calling

Can you use WiFi Calling on a plane?

WiFi Calling pros and cons