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I live in the United States, and have an American phone carrier. At the end of this month, I will go to Japan for about a year. I want to register my phone with a Japanese network. However, I do not want to change my SIM card, since I use my current phone number for two factor authentication on countless services.

Here are the options I can think of:

  1. Keep my current (American) carrier, and pay for roaming. Drawback: This is ridiculously expensive, so I only want to do this if there is no other option.

  2. Get a second phone with a Japanese SIM card. I would still have to use roaming with my first phone (i.e., the current, American one), but I would only use it for two factor authentication. I would use the Japanese phone for everything else.

    Drawback: It's quite unwieldy to have to carry two phones. If I were only staying for a month, I might just suck it up, but a whole year with two phones seems kind of painful.

    Also, I'd have to create a second LINE account for the new phone (since the whole idea is to use the new phone for everything except two factor authentication). I'd have to ask all my friends to add the new account. And then when I leave, I'd have to ask any friends I met in Japan to add the American phone's LINE account. And I would have to repeat this for other platforms beside LINE (pretty much any platform with a "one account per phone" rule).

  3. Switch my two factor authentication to the Japanese SIM card. Drawback: I don't have a comprehensive list of all Internet accounts that I ever opened, so I'm afraid that I might miss some and then get locked out until I return to the US.

All 3 options seem like a major hassle. Am I missing some other option? What do you all do when you go to another country (with different phone carriers) for a somewhat long period of time?

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    Do you have a eSIM capable device? Commented Aug 15, 2023 at 4:33
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    Can your phone use two simcards at the same time? If not, maybe invest in one of those?
    – Willeke
    Commented Aug 15, 2023 at 4:47
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    Change 2fa to use an authentication app? or a security key? There are other options that don't require a sim card. Another option is a virtual number service (google offers one).
    – stanri
    Commented Aug 15, 2023 at 14:03
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    Another option: switch to T-mobile plan that gives you free texting and data and most countries. It's a great option for Google Maps, 2 factor authorization (etc) and WhatsApp but less so if you do a lot of voice calls over the cell network.
    – Hilmar
    Commented Aug 15, 2023 at 16:19
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    @TripeHound Even if the number formats match (I’m not holding my breath) the country code is necessarily different, so keeping the same number is definitely impossible.
    – 11684
    Commented Aug 15, 2023 at 17:27

5 Answers 5

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There's a fourth option: get a phone that supports two SIM cards. This is increasingly common, especially with the advent of downloadable e-SIMs that don't require a physical SIM card slot. It's quite possible if not downright likely that your phone already supports these; for example, all iPhones since 2018 support eSIMs.

Once you have a dual-SIM phone, the rest is easy: slot/load your US SIM in one slot, the Japanese SIM in the other, and configure how you'd like to use them. Typically you'd have the local (Japan) SIM configured as primary/default for making calls and using data at cheap local rates, and the home (US) SIM as the backup that can still receive calls/SMS to your home number.

For maximum flexibility, you'll want to make your home number an eSIM, so you can slot physical SIMs in and out easily as you travel. Most operators will happily do the switch for you for free.

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    Right answer, and exactly what I do (UK/Germany) but note that there are differences in the networks between countries so you also need to check that your phone correctly supports both US and Japanese networks (they may be the same, I don't know). Commented Aug 15, 2023 at 16:58
  • I would actually use the US one as a proper SIM and the local one as the eSIM. The reason behind it that if there's any technical problem with either the phone or the eSIM itself (I had two eSIM settings die and had to ask for new ones; never had an issue with my SIM though it is more than 10 years old) it is easier to swap the foreign SIM into a new phone and obtain a new local eSIM for the new phone. Moving eSIM between phones while possible is more cumbersome, and can involve your carrier needing to send you a new eSIM which might not be possible as you are in a different country.
    – SztupY
    Commented Aug 16, 2023 at 11:44
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    @SztupY eSIMs are usually "sent" as QR codes, so being overseas is no problem as long as you can get access to your email. Of course, if your email is behind a SMS 2FA, that may not be so easy... Commented Aug 16, 2023 at 12:08
  • @lambshaanxy UK providers (at least the few I'm in contact with) send these through the post for "safety" reasons. I have never received them through email or other digital sources and even during Covid restrictions once they tried to ask me to actually head into a shop to pick up the QR code on my own but then later relented and sent it out via post.
    – SztupY
    Commented Aug 16, 2023 at 16:40
  • For what it's worth I was traveling in Japan for the last couple of weeks, and the eSIM option (in addition to my regular US SIM) worked great for me. It was easy to setup, inexpensive, and arrived in my email just a few minutes after ordering it. If you're an Apple user, pretty much all their phones for the last several years support them.
    – Curt
    Commented Aug 17, 2023 at 20:05
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When I went to Japan, I found it very convenient to use a pocket Wi-Fi service: I rented a portable Wi-Fi router which allowed me to have internet access (including VoIP service) with my personal phone wherever there was mobile network coverage.

Since I was traveling by bike in remote areas, it was much more convenient than having to find a combini offering free Wi-Fi.

You can order it from abroad, pick it up at your arrival airport and drop it in a mail box on the day of your departure (or when you decided to stop the rental), using the retour envelop that you will receive with the router. If you are traveling in a group, more users can hook to the same router, making it more convenient.

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    This doesn't help the 2fa though, if he needs to receive a text to a phone number.
    – stanri
    Commented Aug 15, 2023 at 14:04
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    @stanri: It can partially help, because you can keep your original SIM in the phone, and having local wifi means you're not paying roaming charges for data.
    – psmears
    Commented Aug 15, 2023 at 15:18
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    @stanri many US carriers offer "WiFi calling", which includes SMS over WiFi without an active cellular connection.
    – lights0123
    Commented Aug 15, 2023 at 16:57
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    I've done this myself as well, but is it practical for a whole year? I've only seen places that rent them for short trips, and they're not a trivial cost per day
    – Pyritie
    Commented Aug 16, 2023 at 16:56
  • @Pyritie Exactly what Pyritie is saying, "Pocket Wifi" is consistently and by far the most expensive option, then in most countries esims come second, and local physical sim cards are cheapest most of the time. (Japan and South korea being notable exceptions, because roaming on some foreign esims is cheaper than local sim cards...) Commented Aug 17, 2023 at 16:00
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Some other approaches:

  1. Similar to lambshaanxy's answer, but instead of needing a phone with dual SIM, just get an extra non-smart burner phone purely for SMS 2FA use. Put your US SIM in that and block voice calls (or redirect them to a VoIP DID that forwards to your Japanese number for a few cents per minute) and use it just as an authentication device like you would a dedicated 2FA device.

  2. Port your number to Google Fi where you get worldwide roaming at the same price as local. Of course this requires you give Google control over your number, which is rather uncomfortable to a lot of people, but it may be an option for you.

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    Google Fi is a great option, but be warned that Google has reportedly started cutting off service if you're out of the US for more than 3 months at a time.
    – josh3736
    Commented Aug 15, 2023 at 18:18
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Buy another phone?

You could pick up a cheap (used?) smartphone to plug your USA SIM into while you are in Japan, so you could still do 2FA via roaming. Meanwhile acquire a Japanese SIM to do everything else while in Japan.

This is sub-optimal if you need to use 2FA "on the go". But if you only need it while you are at home/ hotel / office, you only need to have the Japanese-SIM phone in your pocket.

Upgrading to a dual-SIM phone would be neater if you are likely to be upgrading your phone in the near future regardless.

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I think the easiest option is to use Google Voice. Transfer your US number to Google Voice and install the app on your phone. Then you can install a Japanese SIM card and still get US texts.

Note that, while I'm currently doing this, both numbers are US numbers and I'm in the US now. But I'm fairly sure that as long as you have internet this will work, although you might also need to use a VPN to make it appear as if you're in the US.

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    To the best of my knowledge, Google Voice has a rather short time limit (I think it was either 60 or 90 days) on how long you can be out of the US before Google suspends your plan. So this does not seem to be a viable long term solution (I'm staying for 10 months).
    – Kyle Lin
    Commented Aug 16, 2023 at 23:21

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