Can WiFi Calling receive SMS? Getting OTP codes and texts over Wi-Fi

The question Thai travellers ask most before a trip is not about calls, it is about OTPs. Pay by credit card online, confirm your banking app on a new phone or log in to a government app, and the code arrives by SMS on your Thai number. This article explains how far WiFi Calling helps with SMS, where it stops, and what to check when a code does not come through.

Can WiFi Calling receive SMS?

Yes. Once your Thai number is registered for WiFi Calling with its carrier, ordinary SMS, bank OTPs included, are delivered to the phone over Wi-Fi, in Thailand or abroad, with no mobile signal and no roaming needed.

The word registered is the whole point. Flipping the WiFi Calling switch on the phone is not enough. The phone first has to build an encrypted tunnel to the carrier's network. When that succeeds, an iPhone shows Wi-Fi next to the carrier name or in Control Center, and Android usually shows a phone-and-Wi-Fi icon or the word VoWiFi. Until you see that indicator, SMS will not come in over Wi-Fi.

User reports on Pantip and Thai travel blogs point the same way: once the VoWiFi indicator is showing, OTPs usually arrive normally. There are reports of codes not arriving too, and most of them trace back to a phone that never registered, WiFi Calling not being switched on before leaving Thailand, or the Thai line being switched off entirely. The Thai carriers' help pages we opened do not explicitly guarantee OTP delivery over WiFi Calling, so treat it as something that works in most cases but that you must test yourself.

What WiFi Calling is and how it works (main guide)

How does SMS travel over Wi-Fi?

In technical terms WiFi Calling is VoWiFi (Voice over Wi-Fi), a 3GPP standard. When it is on, the phone joins an ordinary Wi-Fi network and builds an encrypted IPsec tunnel to a carrier device called the ePDG (Evolved Packet Data Gateway). Authentication uses the secret keys in your SIM (EAP-AKA), the same way it does on a cell tower. The phone then registers with the carrier's IMS (IP Multimedia Subsystem), the same system that runs VoLTE.

Once the phone is inside IMS, SMS can be carried as IP data too. This is called SMS over IP (SMSoIP). On the carrier side a function called the IP-SM-GW takes a message from the traditional SMS centre (SMSC), turns it into an IMS message and sends it down the same tunnel to your phone. Nothing changes for the sender. The bank sends to your Thai number as usual, and the carrier's network decides whether to deliver it over a cell tower or over Wi-Fi.

The GSMA, the global mobile operators' association, publishes a profile called IR.51, IMS Profile for Voice, Video and SMS over Wi-Fi. The title makes it plain that SMS was part of the Wi-Fi service from the start. The practical conditions are therefore the same as for calls: the carrier must enable the feature for your number, the phone must support it, the SIM must be active, and the Wi-Fi must not block UDP ports 500 and 4500, which the tunnel uses.

Can you get Thai bank OTPs abroad over WiFi Calling?

In most cases, yes. A bank OTP is a normal SMS sent to the number tied to your account or card. If that number is registered on WiFi Calling, the code arrives over Wi-Fi. The bank neither knows nor cares which country you are in. What you need to watch is timing. Most OTPs expire within a few minutes. If your Wi-Fi drops at that moment, the network will retry delivery later, but a code that arrives late may no longer be valid.

Travellers usually run into SMS OTPs in these situations:

• Paying online by credit or debit card, for example booking a room, buying a ticket or paying a bill, when the merchant asks for 3-D Secure confirmation.
• Registering your banking app on a new phone, or reinstalling it, which usually needs a code sent to your registered number.
• Logging in to other services tied to your Thai number, such as e-wallets, shopping apps, government apps, or email accounts that use SMS for two-step verification.
• Resetting a password or unlocking an account that has been temporarily locked.

Since 2025 the Bank of Thailand has applied tighter mobile banking rules, including a requirement that banks stop putting links in SMS and emails that ask for sensitive data, and a limit of one banking-app account per device. In practice, if you change phones or reinstall a banking app during a trip, you may have to go through the full verification again, and some steps need an OTP or a face scan. Get this done in Thailand before you go. And any SMS that claims to be from your bank but contains a link should never be tapped, whether it came in over Wi-Fi or over a cell tower.

How to receive Thai bank OTPs while abroad

Does sending SMS over WiFi Calling cost money?

Yes, but not at roaming rates. The simple rule is that WiFi Calling makes the network treat your phone as if it were in Thailand. A text to a Thai number is therefore billed like a domestic SMS under that number's package. If your package includes SMS, it comes out of that allowance. If it does not, you pay the package's normal per-message rate.

• Receiving SMS: Thai mobile packages do not normally charge for incoming SMS, and receiving over WiFi Calling adds no roaming charge because the line is not attached to a foreign network.
• Sending SMS to a Thai number: billed like a domestic text under your package.
• Sending SMS to a foreign number, including numbers in the country you are visiting: billed as an international SMS sent from Thailand, which costs more than a domestic one.
• If WiFi Calling is not registered but the Thai line has roaming on and is attached to a foreign network, the text goes over roaming and is billed at roaming SMS rates instead.

We do not quote a per-message price here, because it differs by carrier and by package and changes over time. Check the myAIS app or your True app, or call AIS on 1175 or True on 1242. The two things to check are how many texts your package includes and what the international SMS rate is.

What WiFi Calling costs for calls and texts

How is iMessage different from SMS, and does it involve WiFi Calling?

Many iPhone users think they have been sending SMS over Wi-Fi for years when it was actually iMessage. Apple explains that iMessage appears in blue bubbles, travels over Wi-Fi or cellular data between Apple devices, and is end-to-end encrypted. SMS/MMS appears in green bubbles, needs a text-messaging plan from your carrier, and is not end-to-end encrypted. Since iOS 18 there is also RCS, which can send high-resolution photos to Android phones, but only where the carrier supports it.

The key point for travellers: iMessage does not need WiFi Calling at all. Any internet connection will do, hotel Wi-Fi or a data eSIM. But bank OTPs never arrive by iMessage. Banks always send them as green SMS, so they depend on your Thai number being registered on WiFi Calling or on roaming. iMessage working fine does not mean your OTPs will arrive.

One setting is worth knowing. On iPhone there is a Send as Text Message option under Settings > Messages (on iOS 18 and later, Settings > Apps > Messages). When it is on and an iMessage fails, the phone resends it as a paid SMS. Abroad this can produce texts you did not mean to pay for, so switch it off during the trip if you want to control costs. On Android, Google Messages and Samsung Messages show the message type in the compose box, such as SMS or RCS.

What are the limits of SMS over WiFi Calling?

• WiFi Calling has to be switched on while you are still in Thailand. True's help page says to enable it before you travel. Turning it on for the first time after you have left often fails to register.
• The SIM must be active and not suspended. If a prepaid line runs out of validity or a postpaid bill is overdue, SMS will not arrive however good the Wi-Fi is.
• SMS only arrives while the line is registered. If you leave Wi-Fi with the Thai line's data off, messages wait at the SMS centre and are retried when you reconnect, which can be too late for an OTP.
• MMS usually needs the line's own mobile data, so it often fails in Airplane Mode. Long texts, texts with pictures and group texts may be converted to MMS.
• USSD codes, such as balance checks or package sign-ups, often do not work over Wi-Fi. Use your carrier's app instead.
• Some Wi-Fi networks, such as public, corporate or captive-portal Wi-Fi, may block the ports WiFi Calling needs. True itself warns that free Wi-Fi in some countries may not support WiFi Calling.
• Some countries or networks restrict internet calling, and Thai carriers publish no official country list. If you are going somewhere known for blocking VoIP, have a backup verification method ready.

Can you receive SMS when using a travel data eSIM alongside your Thai number?

Most travel eSIMs, including esimonline's, are data-only with no phone number, so they have no SMS of their own. Every OTP still goes to your Thai number. The question is whether that number can stay registered on WiFi Calling when there is no Wi-Fi. There are two setups.

For Setup B on iPhone, make the eSIM your data line under Settings > Cellular > Cellular Data and turn off data roaming on the Thai line. Apple notes that on a dual-SIM iPhone you can switch numbers before you send an iMessage, SMS/MMS or RCS message, so when you reply to a text, check that the Thai number is selected. A message sent from a data-only eSIM that has no number will not go out. The quick test is to turn Wi-Fi off, check whether the Thai line still shows the WiFi Calling indicator, then ask someone at home to text you once.

See data eSIMs for travel

How do you test SMS over WiFi Calling before you travel?

1. 1Turn on WiFi Calling for the Thai lineiPhone: Settings > Cellular (or Mobile Data) > choose the Thai line if you have two > Wi-Fi Calling > turn on Wi-Fi Calling on This iPhone. Samsung: Phone app > three dots > Settings > Wi-Fi calling. Other Android phones usually have it under Settings > Network & internet > SIMs > choose the SIM > Wi-Fi calling. Menu names vary by model and carrier, and AIS on Android may first need registration with *399*1#.
2. 2Turn on Airplane Mode, then turn only Wi-Fi back onThis simulates being abroad while you are still at home. Wait until the Wi-Fi Calling or VoWiFi indicator appears in the status bar, which can take a minute or two.
3. 3Request one real OTPUse a service that can send an OTP without a real transaction, such as a forgotten-password flow in an app tied to your Thai number, and see whether the code arrives within seconds.
4. 4Send one SMS outText a family member's Thai number. On iPhone, make sure it is a green bubble, not a blue iMessage, so you know you are testing real SMS.
5. 5If it fails, change the bank's verification before you flyContact your carrier to check that WiFi Calling is enabled on the number, and set your banking app or credit card to confirm in the app instead of by SMS if the bank offers that option.

What should you do when an OTP does not arrive abroad?

Work through these in order, from the easiest fix to the hardest. Do not keep hammering the resend button, because some services lock the account for a while after several requests in a row.

1. Check the status bar first. No Wi-Fi Calling indicator means the line is not registered and SMS will not arrive.
2. Toggle Wi-Fi, or Airplane Mode, once so the phone registers again, and wait a minute or two.
3. Try a different Wi-Fi. Hotel Wi-Fi with a login page or public Wi-Fi may block WiFi Calling. Try a hotspot from another phone, or use your data eSIM instead.
4. Check that the Thai line is still switched on and not suspended. Look at a prepaid line's validity date, or any unpaid postpaid bill, in your carrier's app.
5. If you use two SIMs, check that the Thai number really is the one registered with the bank and that the line has not been turned off.
6. Ask for the code another way. Many services offer in-app approval, email, or an automated voice call, and a call can be answered over WiFi Calling too.
7. If it still fails, contact your carrier (AIS 1175, True 1242) through in-app chat or by calling over WiFi Calling to check the number's WiFi Calling and SMS provisioning, or switch roaming on briefly to receive an important code, accepting your carrier's roaming rates.

One small extra for Apple users: with Text Message Forwarding on, SMS that reach your iPhone also appear on your Mac or iPad, which helps when you are paying on a computer. The message still has to reach the iPhone first, though, so this does not fix an OTP that never arrives.

WiFi Calling not working? How to fix it