Ok, so I set this up and it works just as you described by selecting a non-roaming network from the list. However, it is a battery killer! Like serious battery drain. I can't even get a full day out of it and I can easily go two, three, or more days between charges. I checked my battery usage and No Cell Coverage is a whopping 76%! The googles tell me everyone has this same problem when there no cell service to connect to. While this is a great feature, the battery drain is a deal-breaker for me. I will have to figure something else out.

I am guessing that I don't have that problem because I am using a carrier block (T-Mobile) on roaming charges vs. an active voice line seeking roaming service. Check your voice carrier settings for a similar carrier block. My theory is that if your phone can find the strongest local network but cannot accept chargeable service from that carrier, the battery usage should be normal. I have done this for a few years throughout the world with good results. Alternately, carry a small accessory battery pack when you are out for the day. I do use an iPhone 16 Pro Max, which has remarkable battery life however.

Here is a copy/paste from the individual lines International Settings toggle on my T-Mobile account:

"Block International Roaming (including Charged International Roaming)
Blocks International Roaming for the selected line, except when in Canada and Mexico. If you select this service you will no longer be able to call, text or use data for these lines while roaming internationally; Canada and Mexico excluded. You can use Wi-Fi for calls and text; additional charges may apply."

Thanks for all the help. I don't want to block international data though. That is why I have T-Mobile - for the free international data when I am traveling. I just don't want to get dinged for incoming calls from the US if I am away from wifi and using international data. I wish you could selectively block only voice calls when roaming. Or I wish there was a selection to block only charged international roaming.

The data roaming setting doesn't turn off roaming. It just makes the phone not use data when roaming. It doesn't prevent the phone from registering on the foreign network. There is no "disable roaming completely" setting on iPhone aside from manually selecting a network that your carrier doesn't have a roaming agreement with, which accomplishes the same thing.

If iMessage stopped working, it's because you didn't have a data connection - probably because you turned off data roaming.

I agree with gfunkdave on this point.

A few years ago when I began experimenting with IMS, I tested the effect of the Data Roaming switch. It did not seem to matter if it was On or Off for the main voice line as long as that line was on a non roaming carrier.

For my last overseas trip, I enabled the "Block International Roaming" option on my main line, but kept a second free T-Mobile line from the 2020 Free line events with the roaming enabled. That worked seamlessly. I only turned on the second line if I didn't have a prepaid eSim from wherever I was going.

For the second line, I did an unconditional forward to google voice, so I avoided any charges (not that I think I had any).

Thanks. tried it again with manual network selection, picked something like 520 02 for the network (something I didn't recognize that wasn't AIS or DTAC), and I turned on data roaming on theTrue-H line and now iMessage seems to be working. Hmmm. Did you mean data roaming on the line with the active data connection (true-h, in my case)?

In contrtol center:

https://cimg8.ibsrv.net/gimg/www.fly...2eae666c6.jpeg


And at least I can Message myself and it's still blue this time.

-David

I've been using this configuration for the past couple of years on T-Mobile also. The combination of a dual sim phone with a cheap or free second line for data seems to solve the common issues experienced by many travelers. I really like the ability to have my main voice line active constantly without fear of billing run ups.

IslesFan,

You made no mention of any excessive battery depletion with your current setup. Can we assume you had acceptable battery life with two active lines as I have experienced?

I don't think the battery life was the same as when I'm home domestically, but some thoughts:
1. When I'm out and about in a foreign country, I'm using my phone alot more (maps, etc).
1a. I always had a battery pack with me.
2. When I was working in foreign country (actually at work), my phone was kept on a charger.
3. Doing the "Block International Calling/Roaming" option vs the other way of forcing "Over cellular data" (I.e. manual network selection to a non-existent network or network that the SIM could not roam on anyway) was definitely friendlier to my battery.
4. I did two trips this year to the same place, the first trip where I did the Manual Network selection, and the second where I blocked international roaming, and my battery was better on the second trip.

All in all, the block international roaming/second line is going to be my goto behavior for future trips.

I certainly agree with your experiments. I suspect the fact that running two lines regardless of whether they are on an active network or not, must require more battery power and drain than a single line. It would also seem that if the voice line were attempting to obtain service, it would be using more power to ping the local networks.
The roaming block has been bullet proof from my trials also. I usually just leave it on since it has no negative effect while I am home with local US T-Mobile service. This may also reduce my battery drain while I am Stateside and away from WiFi.

Now I need to figure out how to do the same block international roaming option for AT&T. I have a work colleague who I've lent one of my free T-Mobile lines to, and I haven't figured out how to do the same trick with her ATT line without paying at least for 1 day of the travel pass.

Have you looked at whether this works?
https:/discussions.apple.com/thread/253660132?sortBy=best

*Note, I live outside the US, I have no personal experience / knowledge with AT&T. Plus, who knows if / when AT&T might change how this works…

I did not, but I sent it to my co-worker, so If I have an update, I'll post it. Thanks!

Yeah I know I'm reviving an old thread but maybe an interesting data point:

I've been in the UK for about 3 weeks now, I purchased a LycaMobile 1 month unlimited eSim with my iPhone 17 Pro Max. Did the "Block International Roaming" thing as I did last time for my US T-Mobile line.

This time, I'm noticing what looks like the cellular modem crashing often (maybe once or twice an hour). Don't remember seeing it on my iPhone 16 Pro Max.