A Question About IMS Use Internationally
 

I'm finally able to plan a few trips overseas that might actually happen in the next 12-18 months. I will be traveling with others using dual sim iPhones. Those who use these may know that they can be set up to make and receive free voice calls to and from the US on the T-Mobile network. This is easily set up using your voice line and a separate data line (Via IMS Service) or WiFi calling. I am wondering if two phones with this configuration will allow for free calling between the phones while both are roaming internationally? It seems as if it should.

For example, phone 1 is in Rome, phone 2 is in Athens. Both have a WiFi connection permitting free US calling. Would phone 1 be able to call phone 2's US number and have that ring through to Athens without any carrier charges? If so, would the delay of a round trip signal be a deal killer?

I plan to just leave my phone configured to IMS 24/7, so I get all incoming calls to my US number while roaming. Airalo or other data line services make this easy and cheap. It occurred to me that another phone set the same might be able to connect both for free also.

Just a quick followup, I put 2 iPhones in IMS mode while here in the US and was able to dial each from the other. The time for the ring to appear on the called phone was just 1-2 seconds. That seemed completely normal to me. All that means is there is no block on this procedure, and there is also no delay in the call routing, at least locally. It is encouraging to say the least. It will be very nice to have our T-Mobile phone numbers be active around the world without roaming fees. I'm not sure what to try next, but if anyone has any ideas, I'm glad to experiment.

Following

It just struck me that there may be those who are not familiar with the IMS feature available to dual sim users. IPhones from the XS/XR model and newer all support dual sim if on iOS 13 or newer. If that is the case, just post a request for a description or procedure. It's fairly simple and also works similarly with a second device such as an iPad or companion phone.

I have a Pixel 3a running Android 12. Has both esim and sim slot.

Love the idea of phone to phone while international ( as you described above)

Think it will work for me? What’s the official name of that TMobile sim for travelers?

Bukzin and others, the IMS setup is easy and foolproof on the iPhones I've noted. I do not have a dual sim Android phone, but the general principle should apply if the hardware supports it.

The basic configuration is, on a dual sim, dual service device your primary voice line needs to be set to a non roaming carrier so that it receives no service. On T-Mobile in the US under Cellular-Network Selection, I choose 313-100 which is the First Responders Network on AT&T. Your second line needs to be set to a carrier with data service. The concept provides that the voice line with no service defaults to the data line as a pseudo WiFi service and connects as if on a WiFi network. On dual sim iPhones the Status Bar displays "T-Mobile Using Cellular Data" instead of a carrier name. WiFi should be turned off as a precaution in case you might get a temp WiFi connection, which could be a chargeable call.

If you travel with another device it's just as easy to set up. Get a roaming data plan for your other phone or tablet that allows for hotspot. Turn that hotspot on, while putting your primary voice line cellular off, with WiFi on. Connect your voice device to the hotspot, then you should then be able to make and receive WiFi calls to and from the US on T-Mobiles free international WiFi calling. If you travel with a companion, this system will allow you both to use the WiFi if they also have free US WiFi calling from the same hotspot. I have made 20-30 minute WiFi calls from cruise ships in the middle of the ocean with great connections and no charges over the ships paid WiFi service. That service generally runs about $10-$15 a day for unlimited 24/7 packages. I always buy that, so this is just another perk of a package.

I hope this is clear, but I'm glad to add information as requested.

I'm not aware of any special "T-Mobile Sim While Traveling" you mentioned. I believe practically all TM plans have included or as an option international calling over WiFi to and from the US. Check your plan or with T-Force on Twitter to confirm.

Take a look from a post of mine linking to my reddit comments from a couple of months ago:
https://www.flyertalk.com/forum/trav...l#post33663227

Only charge I had in scotland was when I tried calling a UK number from my T-Mobile line. I do have the stateside international dialing, but it knew I was calling from the UK.

I have been giving this very issue some thought, and might have a partial solution.

I suspect that while roaming internationally it should be possible to make IMS calls to a foreign travel location IF the number called is a toll free number from the US. Many businesses have 1-800 numbers for those in the US, and they might be available with a quick web search. These could be hotels, travel agencies, attractions such as museums & tours, transportation like shuttles that require reservations, etc. You will probably still pay $.25 a minute to order a pizza or call a cab, but at least you can hopefully save on the type I have mentioned. I believe my phone should register as if I am calling from the US and therefore the 800 calls should not be charged. Perhaps a search before departing to create a list of these possibly needed numbers could be time well spent. It might work, it might not. Only one way to find out.

iOS has that feature (Dual sim dual volte or DSDV), so that volte runs over the data connection if the other sim

Don't think pixel 3a has that. This is one of the niche underreported features, often confused with simple dual sim dual standby, and only 3 use cases

1) wanting to use data on one sim but voice on another, or talking on one sim and wanting to receive calls on another sim.
2) LTE only provider like Jio in India, paired with another sim
3) wanting to route calls via data connection on another sim (typically when travelling intl, or in areas where one sim has signal but not another)
https://www.reddit.com/r/pocophones/...gt9o/_/emceri0

Won't help you if it's a local(or a friends cell/mobile) number.

Certainly not, but for the types of numbers I specified, it is possible. I would recommend buying a sim like "3" for $17 on Amazon that provides for 4Gb data, unlimited voice & texts for 30 days if your destination is on their list of roaming countries. Or a local sim with voice service as many have suggested.

Already posted this at Howardforums.com

I’m in the Caribbean now and testing the dual sim IMS function in my iPhone 13 Pro Max. It is not going at all as I expected. In the US, your normal voice line remains in the voice position and a second line is needed as the data provider. To enable IMS, your voice line needs to be set to a non service carrier, then it will default to the data line as a simulated WiFi connection. So far this has not worked using a free line in the data position. 3 of my free lines are not getting data service on any carriers out of about a dozen at 3 locations. However, they do work as voice lines, so I have reversed the roles and am using my daily voice line in the data position, and the free line as the primary voice number. There it is set to a non roaming carrier, and falls back on the voice number data service for IMS. Test calls have all registered on my account as WiFi, even the round trip to my wife’s iPhone with the same setup here.

I contacted T-Force on Twitter and they are also baffled, but opened a ticket for next level support to explore the issue. The free line no data is odd because all my free lines have 1 Plus Promo on them and state 256k free data roaming in the account manager. It should work, but we’ll see.

A drawback to this workaround is that your regular voice line may get charged for incoming calls from the US since it should appear to be actively roaming, however I’m not positive about that while it is set to data service. I have recently been running tests dialing out on the free line set to voice service in the hope that it will not run up a tab @ $.25 a Minute.

One thing you might try, if you have access to your T-Mobile account, is to change your line's international roaming settings under Account / Profile. There is an option there to block either charged international roaming (e.g. cruise ships) or all international roaming (except Canada and Mexico). The pop-up description for the latter feature is that all talk, text, and data on that line will not work outside of the U.S., Canada, or Mexico except if the line is in use on wifi calling. I keep meaning to try this--finally remembered to set a spare line like this for my next trip, thanks for the thread as reminder--to enable calling-via-second-SIM-data so I can't confirm how it will work for you but I think it's worth a shot.

I have been experimenting with configurations in Mexico and IMS is working as I believe it should here. I have my voice line in the voice position with an esim free line set to data. With a non roaming carrier for the voice number I get “No Service Using Celllar Data” in the Status Bar. This gives good data speeds and normal calling functions on both numbers. I’m wondering if perhaps 2nd tier Tech Support fixed this issue (Free Lines Had No Data Roaming Internationally) without notifying me after the trouble ticket was submitted? I realize Mexico is included in the T-MOBILE Roam Like Home feature, but my free lines did not get data upon arrival then mysteriously began to work hours later.

Anyhoo, it works perfectly and I find I can get great data speeds and voice service on both lines with texting also. I also discovered that call forwarding works while set to IMS configuration including texts. So that is a workaround if needed from my experience of switching voice/data lines to provide round trip free calls to traveling companions.

This is hard to describe easily, but pretty easy to set up, so I am glad to explain it in detail via a private message for members.

tldr; You can use IMS function to make free calls to the US & travel companions while traveling internationally. This works on a dual sim phone with dual service. One line for data, one for voice. The Apple Watch Walkie Talkie feature works also.

I upgraded to an unlocked Pixel 6 on Google Fi running Android 12. It is a dual SIM phone. Would love to do as folks on iOS can. Any tips on how I might proceed? Thx