Apple's new dual-SIM function, which lets iPhone XS, XS Max, and XR owners use two cellular subscriptions at once, will come to new phones today. But the current implementation will have a huge negative impact on Verizon subscribers who choose to use dual-SIM in the US, according to engineers who have seen early builds of the software.

Dual-SIM, a popular feature in the rest of the world, is largely unknown in the US. Generally, it's used for three things: roaming internationally, where you get a foreign SIM and also keep your local number; having home and work lines on one phone; or trying out multiple domestic services to see which one is better.

Apple's dual-SIM relies on one physical SIM and an "electronic SIM" or eSIM, which is activated from a menu or an app. AT&T, T-Mobile, and Verizon promised eSIM support at launch, but neither Verizon nor T-Mobile will support the eSIM right now. T-Mobile is working on it and will support eSIM when its software is ready, the carrier told me.

With dual-SIM, a "primary" SIM is used for voice, text, and data, and a "secondary" SIM is used for voice and SMS/MMS on a second subscription.

Apple has a third setting that uses the primary number for iMessage and FaceTime and the secondary number for data, but it isn't clear to me if those services are actually using the primary line for the FaceTime bearer data, or they're using the primary identity over the secondary data connection—that's something we'll have to check.

You can swap those positions at will. AT&T's and T-Mobile's systems work fine in either the primary or secondary positions, supporting those carriers' voice-over-LTE networks.

Hang Tight Until Year's End

The problem comes when a physical Verizon SIM is pushed into the "secondary" position while the phone is in the US. Under the current software build, that kicks the Verizon connection down to 2G CDMA, an old network with significantly less coverage than the current LTE network. The old network also has no MMS support, and certainly wouldn't work for FaceTime. So Verizon customers may find they have perfectly good coverage with their SIM in "primary" position, but no signal and fewer features with the SIM in "secondary."

When I asked Verizon about the CDMA network, the company said that 30 percent of its cell sites were now LTE-only, so there would be a definite coverage hit.

"Wireless customers will be able to activate Verizon service eSIMs as soon as we're confident you'll be able to have the great, high-quality service you expect from us on both your primary and secondary line. If you are a Verizon customer and you activate another carrier's service on your iPhone's eSIM, your Verizon service will be degraded due to the current software configuration," the carrier said.

"Based on our discussions with Apple, we believe these concerns will be resolved quickly and you should be able to add our great Verizon service on your secondary line before the end of the year. Until the secondary line can deliver Verizon's full suite of voice and high-speed data services, we won't activate Verizon service on any eSIM. This includes our own customers' iPhones with dual-SIM capabilities as well as iPhones on competitors' networks."

Apple did not immediately respond to a request for comment.

Roam, If You Want To

The Verizon dual-SIM bug doesn't affect international roaming. If you want to activate a foreign eSIM carrier while abroad and push your Verizon SIM into secondary status for the duration, it'll work fine. Just remember to return the Verizon SIM to primary status when you get back to the US.

As for having a home and work number, right now there are less expensive ways to do that than signing up for an entire second mobile subscription—the apps Sideline and Line2 come to mind; they generally cost about $10 per month.

The dual-SIM feature also may increase carrier switching, though, which is probably one reason carriers are so hesitant. All our carriers have a 14-day grace period for new subscribers, and dual-SIM means you could test out a new carrier's coverage for a week without losing your old number.

Apple's implementation, dual-SIM-with-eSIM, is brand new. When the feature was announced in September, carrier execs I spoke with seemed surprised. They didn't have software or service plans ready, making it sound like Apple had sprung this tech on them just in advance of the launch, and that they were scrambling to catch up.

Once all the bugs shake out, this is going to be great for US mobile subscribers, especially those who roam internationally frequently. For now, though, we'd recommend holding off until they figure all of this out.

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