Travel eSIMs and iMessage/RCS

[japtor]

There's this Apple doc on dual SIM use, but just want to make sure on certain details:

https://support.apple.com/en-us/109317

I'm going on a little trip and figuring I'd save some money with a cheap data only eSIM. But want to still keep my phone number active for emergency calls or texts (...or particularly annoying 2FA/security messages that only go through SMS).

Main thing I'm wondering is if someone iMessage or RCS capable messages my normal phone number, would the messages be delivered through my travel data only SIM, or through my normal SIM over SMS? Same thing goes with outgoing messages. The Apple doc just mentions selecting which number to send with, but can't tell if it'd route through the travel SIM (if iMessage or RCS of course). I'd have my normal number have data disabled and data switching turned off, so theoretically anything data should go through the travel SIM (going by the Apple instructions at least).

 

[kefkafloyd]

I did this in Japan last year. Had data roaming off, data disabled on my US line, and wifi calling enabled for my US line. iMessage will use whatever data coonnection you have. Didn't get a single overage on AT&T. Receiving SMS was free but sending would've incurred a charge.

I don't know about RCS but since it's a data connection I don't see why it wouldn't behave the same way.

 

[japtor]

Ah wifi calling was another aspect I was wondering about, do calls fallback to the data/travel SIM? I was never sure if actually needed wifi specifically or just any data connection. I guess SMS technically works over wifi calling too, I tested it out a while back for another trip, so hopefully that'd work over the SIM too if that's the case.

 

[byrningman]

I’m doing this now and it seems to work fine. Crucially, I get 2FA messages destined for my home SIM that does not have data roaming activated. Game changer for me.

 

[kefkafloyd]

Ah wifi calling was another aspect I was wondering about, do calls fallback to the data/travel SIM? I was never sure if actually needed wifi specifically or just any data connection. I guess SMS technically works over wifi calling too, I tested it out a while back for another trip, so hopefully that'd work over the SIM too if that's the case.

Wifi calling only works over Wifi, IIRC.

 

[japtor]

Wifi calling only works over Wifi, IIRC.

Dang ok well that's what I always figured but was hopeful. Calling isn't a huge concern but incoming SMS is still something like 5 cents on Verizon. Not too concerned there since most stuff should be iMessage/RCS but would be nice to not worry about it.

I guess the alternative would be a mobile hotspot and just stay on wifi everywhere?

 

[kefkafloyd]

Dang ok well that's what I always figured but was hopeful. Calling isn't a huge concern but incoming SMS is still something like 5 cents on Verizon. Not too concerned there since most stuff should be iMessage/RCS but would be nice to not worry about it.

I guess the alternative would be a mobile hotspot and just stay on wifi everywhere?

That's what people used to do before these easy eSIMs, yeah.

 

[japtor]

Wifi calling only works over Wifi, IIRC.

Well I just found out something new (...in the middle of the night after a short nap before dinner turned into a few hours of sleep, yay jetlag), while troubleshooting hotspotting I noticed where it'd say the home network or like "[carriername] Wi-FI" when wifi calling is working, it now said "[carriername] using Cellular Data". First I freaked out of course (although by then I already accidentally tripped the travel day pass), but double checking all the settings things were fine.

Looking it up it is apparently doing wifi calling over the travel data SIM as one would hope. But it's also potentially tricky to actually get it going. The home SIM basically has to go into a no service condition to fall back to data calling, either by actual just having no service, or manually picking a carrier that your carrier doesn't have a roaming agreement with. Looks like I have access to four networks, only one where I'm currently at gives no service to get it working. I don't know yet if that's a coverage thing or no Verizon thing yet, but it's a start!

Ideally there'd be an airplane mode per SIM or disable roaming entirely (vs just data) option. If your carrier gives you a way to disable roaming entirely too that theoretically would work too. Not sure I have that option, not offered online so at minimum would probably have to call in and deal with some support rep.

I'm thinking whether I should just turn off the home SIM, but that'd unlink iMessage from my number right? I feel like that'd be a pain in the ass itself with how many iMessage threads I have. And just another random thought, would things fallback to SMS or RCS in that case?

 

[leet]

Can’t you just select the travel SIM for cellular data, and turn off “enable cellular data switching “?

 

[japtor]

Can’t you just select the travel SIM for cellular data, and turn off “enable cellular data switching “?

That works fine for data, but phone calls and SMS don't need or normally go over the "data" network (or channel or whatever the right terminology would be here?). In this case the travel SIM's data connection is purely a fallback mode if the normal carrier (non data) network can't be used.

 

[iljitsch]

Weird that there is no option to turn off roaming period, but just data roaming. IIRC this was possible in the past.

 

[renfield33]

when i tried this - disabling my primary sim did not let me receive imessage sent to my phone number while using the data on the travel eSIM

but i gave up trying to figure anything out after 5 minutes

 

[japtor]

Right, didn't make it clear in the OP but your home phone number working and being linked to iMessage depends on the home SIM staying active. Hence the thread questions and figuring ways around dealing with that. I think I have things mostly figured out, so gonna try to sum things up here slightly more succinctly:

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If you want to keep your number, keep your home SIM active! If you disable it you'll lose the ability to send/receive calls and texts/iMessages through your home number.

The settings to check for before you go are to make sure Wi-Fi Calling is on, and Data Roaming is off. Can optionally set Data Mode mode to Low Data Mode if you want/need to save on bandwidth. Edit: forgot that also further down in the general Cellular settings, there’s Wi-Fi Assist and the iCloud options.

You can get the travel eSIM ready before you go or while on the trip, but iirc you'll have to do a little settings toggling anyway once you arrive at your destination. There'll be some prompts about like primary and iMessage, just keep those to your home SIM. The main thing is to set the travel one as your data connection, and to turn off Cellular Data Switching.

This last part is the possible pain in the ass (detailed in my longer post above). From what I can tell your home SIM connection prioritization (for calls/SMS) goes Wi-Fi > home cellular > alternative data SIM, so the trick is to get your home SIM into a No Service state to have it fallback to your travel SIM.

If you can turn off international roaming entirely (might have to ask your carrier directly) that's the best option. Otherwise you'll have to hope that your carrier doesn't have roaming agreements with all the carriers at your destination. To find out go to your home SIM settings, and under Network Selection turn off Automatic, and test out the carriers there one by one, which may be extra annoying cause the list takes a while to propagate, and I think you might have to back out of that menu to actually have it switch, then wait to see the status in Control Center.

If all goes well your home SIM should display no bars, No Service should pop up, then change to "[carrier] using Cellular Data"...unless you're on wifi, in which case it'll show that instead, so disable it when doing the last step here.
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If all the carriers work then I dunno, you're boned I guess. I have no clue how likely this is. In my case Verizon worked with three out of the four carriers I saw. My slight paranoia about it is making me think about a travel hotspot in the future, theoretically that'll be way easier to deal with. Do any work with eSIMs, and if so how much of a pain are they to set up? Maybe I should just get cellular with my iPad mini next time, since I carry that around with me anyway in my everyday travel bag...not exactly as pocketable as a hotspot though.

 

[iljitsch]

If all the carriers work then I dunno, you're boned I guess.

You should be ok having voice roaming on if you simply don't make or take any calls.