That's so cool! Do you have to turn on a setting or does it happen automatically?

My limited experience is that once your primary voice line loses it's service, if you are iOS 13.XX then it will default to using data from the secondary line. This equates to WiFi calling, so whatever rate your carrier charges for WiFi calling is what you will have. I'm on T-Mobile which does not charge for WiFi calls to/from the US while roaming internationally. I've been experimenting with this in the US for the past month or so, and it is quite reliable, with good quality calling also. I've used T-Mobile, Verizon, and 3 UK service for the data service and all perform equally well.

I have no experience with Android, but I understand the function is similar, isn't it?

It happens automatically You'd probably want to manually select a network for your main provider (T-Mobile in my case) that they don't support, to make sure you have no service. Otherwise it'll latch on to one of their roaming partners and not use the second SIM to simulate Wi-Fi.

I think this feature really makes traveling internationally so much better. Even with the free 256kbps roaming on T-Mobile, it'd be better to get a local data-only SIM and get high speed data much cheaper and still make free US calls/texts on your original T-Mobile number. Best of both worlds

iOS 13 even allows two phone numbers to iMessage so I load an EU eSIM when going to Europe and can use both numbers to message

iMessage will even consolidate texts into one "thread" when a contact sends messages to either number, and send your response from the number the last message came to.

Some people will install WhatsApp Business alongside the usual WhatsApp version, so you can use each one with a separate number.

Which EU eSIM do you use?

Here is a list of esim providers sorted by coverage areas

esimdb.com

Airalo is a eSIM marketplace in the way Amazon is marketplace. Your luck all depends which eSIM you buy from them. esimdb.com seems to be the most comprehensive list of your eSIM options.

I've seen that database but I don't know how often it's updated. I recently became aware of another (Chinese) esim carrier called RedteaGO. Their rates are shown,($10.99) but they are not current from what the iOS app shows for purchase info. For instance, 1Gb of data good for 30 days in Italy or Spain is $4.99 on the App for purchase. They currently have a promo listed that gives you your first purchase for $1 also. I believe the carrier was Vodaphone, but not positive where I saw that claim. I downloaded the app and it's fairly easy to navigate, listing 42 countries and the prices for their plans. They also have regional plans vs. single country only. In defense of the "esimdb.com" site, the prices shown on RedteaGO's official website are the same as esimdb has posted.

I kinda like the option for a low priced, low data plan that is more than a week or ten days long. It suits the requirement for IMS WiFi/voice calling feature. 1Gb of data will power several hundred minutes of WiFi calling spread out over a month while roaming. 36 Euro countries, 30 days is $6.99 for the 1Gb service, 3Gb=$15.99.

It appears the esim wars have begun!

Have you purchased Airalo eSIM?

I have a 2017 iPad Pro (10.5 inch). It doesn't appear on there when I go to Settings App > Cellular Data > "Add a New Plan"

I see US carriers, AlwaysOnline, GigSky, Truephone.

Would it appear if I went to Europe? Or do I need to install the Airalo app?

From what I see on the Apple website, your iPad does not have an esim, but an "Embedded Apple Sim" instead. That's why you can access some carriers mobile plans, but not Airalo. I recently switched from my iPad Mini 4 to a Mini 5 to get the esim. Here's the chart from Apple.com where I did a screen grab:

Ah thanks.

It seems slightly odd to me that the capability an esim provides cannot be engineered into a hardware, simslot sim. Wouldn't that make sense, and generate revenue for an inventor? It would restrict a device to a single carrier at a time, but for travelers, that's not a big deal. Switching between esims is quick, easy and reliable. It seems to me that Apple is half way there with the Apple Sim already.

I have a data-only eSIM in use that was purchased from Airalo. But I�m using this Airalo eSIM on a non-iPad device.

The only Airalo-sold eSIMs that gave me major problems to use were ones that were for to be for US data use. I�ve since come across a possible workaround to the problem I faced with the Airalo-sold US eSIM, but I haven�t tried it out on any devices yet.

From my limited experience, the eSIM from Airalo
(running on ATT) worked fine here in the US.

Used it with my Pixel 3a phone with the Android 10 OS.

I have used an Airalo US esim without any issues, but it was back when they were aligned with AT&T as the roaming carrier. My current carrier is T-Mobile so there is no point in testing the service from Airalo on TM until I am roaming overseas. I will be going to Europe in a few weeks and will probably buy a weeks worth of service for Spain for the 6 days I'll be there though. My primary roaming service is intended to be a "Three" sim from Amazon in the sim slot on my iPhone 11 Pro Max. I suspect I will get the Airalo for either my 11" iPad Pro or Mini 5 for a $3 test. I'm anxious to test the quality and reliability of those 2 roaming services and also the IMS voice function while using those data services. I hope to give some user feedback back here and may be able to field questions or requests while overseas.