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Apple's Dual-SIM Tech Ruins Verizon Coverage (pcmag.com)
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, PCMag reported Tuesday, citing engineers who have seen early builds of the software. From the report: 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.
[...] 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.
[...] 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.
Actually, it is probably with Apple, since the second SIM only supports 2g and no MMS
By that logic, your up-to-date browser is at fault for using an old encryption method to talk with a site that hasn't yet been updated to work with newer encryption.
Just as a browser will fallback to an older method in order to talk to a site that hasn't been updated, the eSIM is falling back to what little support it has until the carriers add full support.
Multi-SIM phones aren't common in the US because of incompatible networks and frequency bands, and a drive (by American consumers) for high-speed mobile data.
Most multi-SIM phones limit one or more SIMs to 2G GSM only, which limits those secondary SIMs to the T-Mobile network in most locations. (There are other 2G GSM networks, but ALL of those others are small regional networks with seriously limited coverage, and T-Mobile already has roaming agreements in place with all of them.)
Getting LTE and CDMA working in the same phone on a single SIM involved enough hacks as it is (this is why Verizon's earliest global phones used the SIM solely for the GSM modem and kept the CDMA modem totally separate, it took OEMs years to work out all the bugs). Getting it working over multiple SIMs is, of course, going to be a nightmare.
The rest of the world avoids this by: