https://www.macworld.com/article/1441708/how-to-use-two-iphone-esims.html [article is followed by Apple Support document https://support.apple.com/guide/iphone/use-dual-sim-iph9c5776d3c/ios%5D

How to use two eSIMs in an iPhone to balance data usage

Apple offers a switch that can help while roaming with networks that have different coverage and data limits.

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By Glenn Fleishman, Senior Contributor, Macworld JAN 12, 2023 1:00 AM PST

Image: IDG

iPhones and iPads that support two cellular calling plans at once add an option that lets your device draw from both connections when it’s useful. iPhones and iPads released in the last few years with a physical SIM slot let you add a second as an eSIM. The iPhone 13 and later can also use two eSIMs instead of a physical SIM, while iPhone 14 models sold in the United States only have eSIM support (for one or two in use at a time).

You might use a second SIM in your home region if each network subscription has different capabilities, such as non-local free voice calling or data limits based on network coverage. However, it’s most likely you’d install another SIM for travel away from your home region or country, as I did recently to Central Europe.

If you have some coverage from your main carrier and use a roaming SIM to complement that, you can take advantage of a feature Apple offers to manage cellular data use. In Settings > Cellular/Cellular Data, you can Allow Cellular Data Switching. Apple describes this feature succinctly as, “Turning this feature on will allow your phone to use cellular data from both lines depending on coverage and availability.” In practice, it works with a fair amount of sophistication, and it’s worth turning on to have what appears to be an optimum experience.

I subscribe to T-Mobile in the U.S. and the company’s Magenta and higher-tier Magenta Max plans include unlimited data at 256Kbps in what T-Mobile describes as 215-plus countries without roaming fees. (Unlimited texting and voice calls at $0.25 per minute are also included.) In 11 countries covered by the Magenta plan and all countries with Magenta Max plans, T-Mobile also includes a total of 5GB per monthly billing cycle at 5G rates. In my travels, that covered three countries out of the five I visited. (Sadly, my billing cycle put all usage in the same month instead of 5GB from T-Mobile in one month and 5GB in the next.)

Apple

In addition, I purchased a 5GB eSIM that worked across Europe. I found Wi-Fi everywhere, too, and hotels, restaurants, museums, and other locations prominently displayed access codes or offered them freely. Relying on Wi-Fi where possible reduced my cellular data load.

At times, I found cell data service lagged even when I should have been getting 5G rates from T-Mobile, likely due to coverage gaps. The same occurred at times while switched over to the purchased eSIM plan.

Enabling Allow Cellular Data Switching offered what seemed like a distinct improvement, providing more consistent coverage at the highest possible data rate without draining either 5GB pot fully.

Apple doesn’t disclose what it means by “coverage and availability” but it was clear that while in areas that T-Mobile offered just 256Kbps, that still supplemented the purchase eSIM–otherwise, I would have drained the 5GB faster.

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On the last day of my trip, I passed through the airport in Montréal. Canada is another country covered in T-Mobile’s Magenta 5GB deal. Checking usage after, I found I still hadn’t run through my T-Mobile data and was at about 4.7GB on the purchased SIM just before taking off on the final leg home. A perfect landing.

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Table of Contents 

Use Dual SIM on iPhone

You can use Dual SIM with the following:

• iPhone XR, iPhone XS, iPhone 11, iPhone 12, iPhone 13, iPhone SE (3rd generation), and iPhone 14 (purchased outside the U.S.); these models can use one physical SIM and one eSIM.
• iPhone 13, iPhone SE (3rd generation), and later; these models can use two eSIMs.

Note: eSIM isn’t available in certain countries or regions.

Here are some of the many ways you can use Dual SIM:

• Use one number for business and another number for personal calls.
• Add a local data plan when you travel to another country or region.
• Have separate voice and data plans.

Note: To use two different carriers, your iPhone must be unlocked. See the Apple Support article How to unlock your iPhone for use with a different carrier.

Set up Dual SIM

1. Go to Settings  > Cellular, then make sure you have at least two lines (below SIMs). To add a line, see Set up cellular service on iPhone.
2. Turn on two lines—tap a line, then tap Turn On this Line.You can also change settings such as Cellular Plan Label, Wi-Fi Calling (if available from your carrier), Calls on Other Devices, or SIM PIN. The label appears in Phone, Messages, and Contacts.
3. Choose the default line for cellular data—tap Cellular Data, then tap a line. To use either line depending on coverage and availability, turn on Allow Cellular Data Switching.You may incur roaming charges if Data Roaming is on and you’re outside the country or region covered by the carrier’s network.
4. Choose the default line for voice calls—tap Default Voice Line, then tap a line.

When using Dual SIM, note the following:

• Wi-Fi Calling must be turned on for a line to enable that line to receive calls while the other line is in use for a call. If you receive a call on one line while the other is in use for a call, and no Wi-Fi connection is available, iPhone uses the cellular data of the line that is in use for the call to receive the other line’s call. Charges may apply. The line that is in use for the call must be permitted for data use in your Cellular Data settings (either as the default line, or as the non-default line with Allow Cellular Data Switching turned on) to receive the other line’s call.
• If you don’t turn on Wi-Fi Calling for a line, any incoming phone calls on that line (including calls from emergency services) go directly to voicemail (if available from your carrier) when the other line is in use; you won’t receive missed call notifications.If you set up conditional call forwarding (if available from your carrier) from one line to another when a line is busy or not in service, the calls don’t go to voicemail; contact your carrier for setup information.
• If you make a phone call from another device, such as your Mac, by relaying it through your iPhone with Dual SIM, the call is made using your default voice line.
• If you start an SMS/MMS Messages conversation using one line, you can’t switch the conversation to your other line; you need to delete the conversation and start a new conversation using the other line. Also, you may incur additional charges if you send SMS/MMS attachments on the line that’s not selected for cellular data.
• Instant Hotspot and Personal Hotspot use the line selected for cellular data.