Dual SIM in Japan: Keep Your Number, Avoid Roaming Fees

Short answer: you can keep your home number switched on in Japan and still avoid roaming charges, but only if you make your travel eSIM the data line and stop the home line from doing anything billable. On iPhone that means setting Cellular Data to the Japan line and turning off Allow Cellular Data Switching; on Android it means setting the Japan SIM as the default for mobile data and leaving roaming off for the home SIM. The part most people miss is that on some carriers simply answering a call can start a daily international fee, so the settings alone are not the whole job.

This guide is about the home line, not the travel eSIM. It assumes you have already bought and installed a Japan eSIM and now want your own number to sit quietly in the background for two-factor codes and the occasional call.

Why a dual SIM setup causes surprise charges

Modern travel roaming is usually sold as a daily pass rather than a per-megabyte rate, and the daily fee starts the moment the phone does almost anything on the home line. That is the trap: you never chose to roam, your phone just did something small on your behalf.

AT&T describes its International Day Pass as starting “the first time you use data, send a text, or make or accept a phone call in an included destination,” at $12 per day for the first line and $6 for each additional line used on the same calendar day (prices as of September 2026). Note the word accept — an incoming call you did not ask for can begin the 24-hour billing window.

Verizon words TravelPass the same way. Its page says a 24-hour session starts when you “make or answer a phone call, send a text or use data,” priced at $12 per line per day in most countries and $6 per line in Canada and Mexico (as of September 2026). Verizon also warns that background data from apps can start a session on its own.

If you skip the daily pass entirely, you fall back to pay-per-use rates. AT&T’s pay-per-use page lists $4 per minute for calls in “all other countries,” $1 per text message sent, $2 per picture or video message, and $2.05 per MB of data (as of September 2026). That page does not break Japan out as its own line item, so check AT&T’s destination lookup for your specific plan before you rely on those numbers. At $2.05 per MB, a single forgotten photo backup is genuinely expensive.

The point is not that one carrier is worse than another. It is that the billing trigger on both is broader than “I opened a browser.”

How to set up dual SIM on iPhone so the home line costs nothing

Apple’s Dual SIM support page covers the settings below. iPhone models support Dual SIM using a physical SIM plus an eSIM, and iPhone 13 and later also support two eSIMs at once.

1. Label both lines first. Go to Settings > Cellular (or Mobile Data), tap a number, then Cellular Plan Label. Apple lets you pick a preset or “enter a custom label.” Naming them “Home” and “Japan” removes most of the confusion in every later step.

2. Point cellular data at the Japan line. Go to Settings > Cellular (or Mobile Data) > Cellular Data and tap the Japan number. Only one line uses cellular data at a time.

3. Turn off Allow Cellular Data Switching. This toggle lives in the same place: Settings > Cellular > Cellular Data > Allow Cellular Data Switching. Left on, it lets the phone move data to whichever line is handling a call — which is exactly the behaviour you are trying to prevent. Apple is explicit about the trade-off: with it turned off, “if you turn off Allow Cellular Data Switching and you’re active on a voice number that isn’t your designated cellular-data number, cellular data won’t work while you’re on the call.” In other words, take a call on your home number and your data pauses until the call ends. For most travellers that is an acceptable price for a predictable bill.

4. Choose a default voice line. Also under Settings > Cellular, you set which number is the default for contacts not already in your address book, and you can assign iMessage and FaceTime to either or both numbers.

5. Know one side effect. Apple notes that if data roaming is on for the number using cellular data, your voice-only number will have Visual Voicemail and MMS disabled. If your home voicemail stops transcribing while you are in Japan, that is why, not a fault.

How to set up dual SIM on Android

Menu names vary by manufacturer; the paths below are the ones Google documents for Pixel. Look for an equivalent “SIM manager” or “SIMs” screen on other brands.

Set your primary SIMs. Go to Settings > Network & internet > SIMs. Under “Your primary SIMs” you set Calls, Texts and Mobile data separately. Google notes that “Only one SIM can be the default for data.” Set Mobile data to your Japan eSIM and leave Calls on your home number if you want to keep receiving calls.

Understand the call exception. Google’s help is direct about this: “Most data goes through the default SIM for that use type. Exception: During a phone call, all data goes through the SIM making the call.” So a call answered on your home line routes data over your home line for the duration, regardless of your default.

Check the roaming toggle per SIM. Roaming lets your phone “send data over other carriers’ networks when you leave an area covered by your carrier’s networks.” On a dual-SIM Pixel, each SIM has its own tab at the top of Mobile network settings, so make sure you are switching roaming off on the tab for your home line, not the Japan one — the Japan eSIM generally needs roaming enabled to work at all.

Automatic data switching. Google documents an “Automatic data switching” option for a second SIM on Pixel 8a and later with dual calls enabled, which allows data to move to whichever SIM is on a call. Like Apple’s equivalent, leave it off while travelling. There is also a “Data during calls” toggle under the same SIMs screen.

One more Android limitation worth knowing: Google states that on Pixel 8 or earlier, if you are on a call you cannot receive a call through the other SIM at the same time.

What still costs money when data roaming is off

Turning off data roaming is necessary but not sufficient. AT&T’s own guidance on managing international usage says that switching off data roaming stops charges for “email, web browsing, Visual Voicemail, and sending or receiving picture and video messages” — but adds that “You should be able to make and receive phone calls and text messages.” Voice and SMS are a separate billing channel from data, and they stay live.

Because the daily-pass trigger on AT&T and Verizon includes accepting or answering a call, the safest configuration is not “data roaming off” alone — it is data roaming off plus a habit of not answering calls on the home line. If your carrier lets you block international roaming outright at the account level, that is stronger than any phone setting.

Does Wi-Fi Calling avoid roaming charges in Japan?

Partly, and it is the single most useful tool here. Verizon’s Wi-Fi Calling FAQ states that when you use Wi-Fi Calling outside the US, “Calling back to the US is free,” while “Calls to any other country will be charged as per your international long distance calling plan. If you don’t have one, you’re charged international long distance pay-as-you-go rates.”

So calling home is the cheap case; calling a Japanese restaurant to change a reservation is the expensive one, because that is an international call to a third country. Use a Japanese number sparingly, or make those calls over the Japan eSIM’s data with an app.

Two cautions. First, Wi-Fi Calling only helps while you are actually on Wi-Fi — if the connection drops mid-call the phone can hand back to the cellular network, and you are roaming again. Second, terms differ by carrier and by plan, so read your own carrier’s Wi-Fi Calling page rather than assuming Verizon’s terms apply to you.

A Japan-specific issue: your home line needs VoLTE

Japan has finished shutting down 3G. NTT docomo ended its FOMA (3G) and i-mode services on 31 March 2026, and its announcement notes that 4G devices without VoLTE support lose voice calling, because those handsets previously fell back to the 3G network for calls.

The practical consequence for a visitor: an older phone whose voice roaming depended on 3G fallback may register for data but fail to place or receive calls in Japan. If you are travelling with a device more than a few years old, check with your carrier that VoLTE roaming is enabled on your line before you leave. This is one reason a data-only travel eSIM plus messaging apps is a common approach — it sidesteps the voice question entirely.

Pre-landing checklist

• Install and label the Japan eSIM before you fly, while you still have reliable Wi-Fi.
• Set the Japan line as your cellular data / mobile data line.
• Turn off Allow Cellular Data Switching (iPhone) or automatic data switching (Android).
• Turn data roaming off on the home line and on for the Japan line.
• Turn Wi-Fi Calling on for the home line so calls home still work over hotel or café Wi-Fi.
• Ask your carrier whether receiving calls or texts triggers a daily international fee on your specific plan.
• Confirm VoLTE is enabled on your home line if your phone is older.
• After landing, check the status bar shows both lines and that data is flowing on the Japan one before you disable anything else.

Frequently asked questions

Will I still get my two-factor authentication codes by SMS?
Generally yes, as long as the home line is still registered on a network — SMS is not affected by the data roaming toggle. Receiving texts is free on many plans, but confirm with your carrier, and be aware that on carriers whose daily pass triggers on texting, sending a reply may start a charge.

Can I just turn off my home line completely?
You can, and it is the most predictable option for cost. The trade-off is that you will not receive SMS codes or calls to your own number. If any account you need in Japan uses SMS two-factor authentication, switch those to an app-based method before you travel.

Why does my data stop working during a phone call?
That is the expected behaviour once you disable data switching. Apple states that with Allow Cellular Data Switching off, cellular data will not work while you are on a call on a line other than your designated data line. Google describes the same effect on Android: during a call, data goes through the SIM making the call. Data returns when the call ends.

Is a daily roaming pass ever the better choice than a travel eSIM?
It can be, for very short trips or for travellers who need their own number to work normally for calls. At the daily rates listed above (as of September 2026), a week of roaming costs meaningfully more than most Japan data eSIMs, but roaming keeps one number and requires no setup. Compare against your actual trip length rather than assuming either is cheaper.

Do these settings work on any phone?
They require a phone that supports dual SIM and is carrier-unlocked. Apple documents Dual SIM with a physical SIM plus an eSIM, with two eSIMs supported on iPhone 13 and later. Android support varies by model and manufacturer, so check your own device’s SIM settings screen before buying an eSIM.

Sources

• Using Dual SIM with an eSIM — Apple Support (accessed 2026-09-03)
• How to use dual SIMs on your Google Pixel phone — Google Pixel Phone Help (accessed 2026-09-03)
• Connect to mobile networks on a Pixel phone — Google Pixel Phone Help (accessed 2026-09-03)
• International Day Pass — AT&T (accessed 2026-09-03)
• International Pay As You Go call, text and data rates — AT&T (accessed 2026-09-03)
• Ways to Manage International Usage — AT&T Wireless Customer Support (accessed 2026-09-03)
• International Plans: TravelPass — Verizon (accessed 2026-09-03)
• Wi-Fi Calling FAQs — Verizon Support (accessed 2026-09-03)
• FOMA and i-mode service termination notice — NTT docomo (accessed 2026-09-03)