Here's how Dual SIM works on the iPhone Xs and Xs Max
For the first time ever, Apple has announced iPhones with Dual SIM functionality. The new iPhone Xs and iPhone Xs Max have this built-in, but in most places it won't necessarily be what you might expect.
See, the iPhones come with just one physical SIM slot as usual. It's just that this is augmented with an eSIM. So if at least one of the carriers you intend to use supports the eSIM standard, then you can take advantage of the Dual SIM function. Otherwise, you're out of luck.
The good news is that more and more carriers should start waking up to the eSIM reality. The bad news is that, at the moment, the list is very short: only 14 networks across 10 countries support this standard, along with worldwide service providers GigSky and Truphone.
Note that to use two different carriers, your iPhone must be unlocked. If a CDMA-based carrier (such as Verizon or Sprint in the US) provides your first SIM, then your second SIM will not support CDMA.
To use the eSIM you will activate your cellular plan with a QR code provided by your carrier. You can store more than one eSIM in your device, but can only use one at a time.
If you buy an iPhone Xs Max in China, Hong Kong, or Macau, none of the above applies. Over there Apple will provide a special SKU of its biggest handset ever, one which comes with a proper Dual SIM setup. As you can see from the image above, the two cards will basically sit on top of each other in the slot.
Both systems use Dual SIM Dual Standby, which lets you make and receive calls on both lines. Note though that if one number is on a call, incoming calls on the other number will go to voicemail. Also, if you are active on a voice number that isn't your designated data number, data will not work while your are on the call.
As with other Dual SIM solutions on the market, Apple lets you label each SIM card, as well as choose a default line. The secondary line can be used for data only - a handy option if you're traveling and pick up a local SIM just for that purpose.
For each contact you call the iPhone will choose the same number you used last time. If you haven't called before, then your default number will be used. Yet if you want you can specify which subscription to use for a specific contact.
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