iPhone settings for dual SIM happiness

I return to the one forum I know of where you can ask a detailed question and receive an answer from contributors who actually read the question. TidBITS Talk seems unique in this respect. So here goes:

I will be travelling (to Japan, but that’s not relevant, from the UK, and neither is that). I have purchased a data-only eSIM to use while abroad—mainly for Google Maps and online guides while out and about. But I also want to be able to receive data-based messages and voice calls (so mainly iMessage, FaceTime, WhatsApp). And most the people who will be sending me those messages and making those calls will be using my regular UK cell phone number. My question is, how exactly should I configure my iPhone 11 settings (iOS 17.4.1) in order to achieve that, while avoiding the risk of using any inordinately expensive data or receiving any texts or calls on my UK cell service.

This is what I think the settings should be, all in the Settings section called ‘Mobile Service’ (which the Apple support site seems to think is called ‘Cellular’). My UK SIM card is ‘Primary’, the Japanese eSIM is ‘Japan’.

SIMS > Japan > Turn On This Line on; Data Roaming on

SIMS > Primary > Turn On This Line on; Data Roaming off

Mobile Data > select Japan; Allow Mobile Data Switching off

Default Voice Line > select Primary

Will these settings allow an iMessage or FaceTime call to reach me, using the data allowance on the Japanese eSIM? Or should I instead have the Primary line turned off, so it’s not in use at all? It seems such a simple question, but one to which I have failed to find a straight answer. If anyone here can answer the question I’d be eternally grateful.

Many thanks

Since no one else has answered, I’ll tell you what I know. I know nothing.

Generally, when I leave the country (USA), I buy an eSIM for my dual SIM iPhone and try to accomplish what you seem to be trying to accomplish. Since I know nothing, I review FlyerTalk’s Technology forum. So far, I seem to have gleaned enough information each time, because I have been able to make local USA calls without incurring any extra charges from my USA provider.

My notes (which I do not update properly) seem to tell me to take the steps that you outlined, albeit in a different order (which I think should not matter) and with different words. The only thing my notes add is to ensure Wi-Fi calling is on for the home SIM. I understand this is necessary for the home SIM to use the data from the travel SIM. (I believe you do not need to turn Personal Hotspot on for the home SIM to use Wi-Fi calling, but you do need to turn on Wi-Fi calling. Corrections are welcome.)

My notes also suggest turning on Low Data Mode, but that is marked optional.

Good luck and please report back, especially if you deviate from what you outlined.

Does your home plan support receiving calls and text messages without a charge when you are out of the country? If yes, this should be ok. If they charge a lot for per-minute receiving of calls or for receiving text messages, then you may want to turn the line off, because you can’t control when somebody tries to call you or text you.

I haven’t been to Japan, so I don’t know if WhatsApp calling is something that is common there, but it is common when I have traveled outside the US and want to make WhatsApp calls (using either WiFi if I am at my hotel, or using mobile data on the foreign eSIM) for making restaurant reservations, etc., if you need to make calls.

Will, thanks so much for responding. Yes, I did intend to keep Wi-Fi Calling turned on on my Primary line—I mean, using Wi-Fi is not a cellular network function, so I would always use Wi-Fi if available. My Personal Hotspot is always turned off. And agreed, for the Japan line I would turn on Low Data Mode, to conserve the data I have purchased on the eSIM.

I will report back about the effectiveness of all these settings.

Hi Doug. Thanks for your response. I would get charged for receiving calls—but I intend not to answer unless I can see, and want to speak to, whoever is calling, As far as I can tell I don’t get charged for receiving SMS messages.

I suppose I could turn the primary line off completely—but my fear of doing that is that I wouldn’t receive iMessage and WhatsApp messages and calls sent to my mobile number. This represents the whole crux of my perplexity: trying to understand what should be on and what off in order to be able to receive these data messages and calls sent to a phone number that as far as I know exists only on the primary SIM—which I don’t want to use! And though this issue must concern most (all?) travellers using an eSIM, all the online sources I have found fail to explain the answer! (By the way, in response to your comment about WhatsApp usage in Japan, I believe WhatsApp is used extensively throughout Asia)

iMessage should be fine, as they also get sent to your Apple ID and are sent using WiFi or mobile data, not voice or SMS. I’m not 100% sure about WhatsApp, as I don’t use it often, but it should be the same. You just need a phone number to set up the service.

But there is a good way to test at home before you go. Turn on airplane mode and keep WiFi active and have somebody send you WhatsApp and iMessage messages, or send them yourself and verify that they are delivered.

It may be, thought, that your phone needs a voice SIM to be active, thought think not. You will see “SOS” where the carrier and signal strength is usually shown, though, I think.

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Several years ago, my cell provider at the time (AT&T) told me that WiFi calling from an international location counted as an International call. I also had a VOIP landline at home with an app on my cell for making calls, so I used that app from my hotel to talk to my doctor about a medical issue that arose.

Same here with T-mobile. When I’m traveling in Japan I get hit with their $0.25/min roaming charge for both incoming and outgoing calls regardless of vanilla cellular or wifi call. Wifi calling is really only about signal quality and coverage for me. In terms of charge, there’s no difference (at least for my T-mobile plan).

And this just for laughs, those $0.25/min roaming charges in Japan are nothing compared to the other way around. Calling to Japan from the States is $3/min unless I subscribe to some foreign call subscription shenanigans. :laughing:

Those should work fine. They don’t send content to you mobile number. They send it to you via an Internet-based server. The phone number is only used to look up your account. Once it’s found, everything is Internet based, whether that’s via Ethernet, Wi-Fi or anything else you can think of.

I’ve read that as well. Go to your service provider’s web site and/or call customer service to see how it works for you. With Verizon, Wi-Fi calls are all billed as if I’m in my home country - they don’t care about where you’re physically located. I used it quite a bit during a trip to Italy and never got billed for any Wi-Fi calls to/from the US. But Wi-Fi calls to/from Italian numbers were billed as if I was back home. I also didn’t get billed for SMS texts which really surprised me until I realized that my account has some unlimited text feature for Canada, Mexico and most EU countries.

UK (and European) mobile providers don’t charge for receiving SMS when roaming – you have no control over whether you receive one or not so it would be unfair to charge for something you haven’t agreed to. Calls are obviously different as you have the choice not to pick up.

I’m not sure you can rely on this 100%. The issue is when does Apple de-register your number from iMessage? Turning on aeroplane mode won’t answer that, as it’s the lookup bit that @Shamino refers to that is at risk. It used to be that as soon as you took a SIM out the number was de-registered from iMessage. But lately that doesn’t seem to be the case (I occasionally need to do some testing that involves removing my SIM). And turning off the line is not quite the same as removing the SIM. But the key to whether iMessage will continue to work if you turn off your primary SIM is to understand when your number gets de-registered from iMessage.

For what it’s worth, I would do exactly this as it will represent the least disruption to your normal setup and should avoid any roaming charges (as long as you don’t answer any calls that come in to your primary SIM). They key to avoiding roaming charges is selecting Japan in Mobile Data with Allow Mobile Data Switching off. In this case the phone will never try to use the primary SIM for data. I use this within the UK as my primary SIM has a low data allowance and my secondary one has a more generous one.

It sounds like you’re on the Apple US support site. If you switch your country region to the UK you will get localised support documents that have terminology that matches the UK English terms in iOS & MacOS.

About mobile data roaming options for your iPhone and iPad – Apple Support (UK)

If you end up on a US (or other) support page, you can either edit the URL and change /en-us/ to /en-gb/ or just remove the language code entirely. In the latter case, Apple will automatically redirect to the local page for the country you are accessing the internet from (which might not be what you want in Japan!). Eg: https://support.apple.com/109037


I’m pretty sure it only happens when you explicitly sign-out from iMessage on your phone (or today, as a part of wiping a phone).

I recall all of the issues people used to have with iMessage content being sent to old phones that were re-sold, leading to Apple telling everyone to explicitly sign-out of iMessage before wiping an old phone.