Embedded SIM (eSIM) is a technology that allows a mobile device to join a carrier's network without the need for a physical SIM card. The first eSIM-equipped device appeared in 2015, but the technology has only started to hit the mainstream in the past year or two.
Every iPhone since the iPhone XR has had support for two SIMs, one physical nano-SIM and one eSIM. All Google Pixel devices since the Pixel 2 include dual-SIM support, as do most recent Samsung Galaxy models and a smattering of phones from other carriers.
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The most obvious advantage of using an eSIM is that you don't have to go to a carrier's store to pick up a physical SIM or wait for one to arrive in the mail. You can sign up for a mobile plan and activate it immediately by scanning a QR code, using a carrier's app, or even manually creating a carrier profile. (Your phone must be unlocked, of course. If you try to set up an eSIM for an alternative carrier on a carrier-locked device, you'll get an error message.)
But having two SIMs available unlocks other useful scenarios. For example, you can use one SIM for a business line and the other for a personal line. In that configuration, you can make and receive calls and use SMS messaging on either number. For cellular data, you have to designate a line, although you can switch lines by flipping a switch in settings. You can even have multiple eSIM profiles set up and switch between them in settings.