WAILEA, Hawaii—Today we got our first view of T-Mobile's eSIM app, which looks like it lets you connect a new prepaid T-Mobile line without going into a store or having a physical SIM card shipped to you. Sign up on your new iPhone with just a few clicks.

T-Mobile is rolling this app out to its staff for training tomorrow and aims to launch it by the end of the year, according to sources close to the development and rollout. For now, it will only support adding prepaid plans to eSIMs; the thinking seems to be that it will be used for inbound roaming and secondary lines, with customers still going into stores and getting physical SIMs activated for primary lines and family plans.

T-Mobile's app-based system is easier than the other eSIM options: a carrier sending you a QR code that needs to be scanned or an in-store activation.

The eSIM functionality in new iPhone XS, XS Max and XR devices lets you add and subtract service plans from different carriers at will, and maintain two active lines at the same time. At least, that's the theory. US carriers didn't support the eSIM at launch. The first support came from roaming carriers GigSky and Truphone, both of which turned the feature on last month.

AT&T confirmed through email that customers should visit retail stores to receive QR codes to activate their eSIMs, and that AT&T will be able to support all prepaid and postpaid service plans for eSIM-based iPhones. Fierce Wireless says Verizon will begin to support eSIM tomorrow.

What of Sprint? At Mobile World Congress Americas, reps said the company will support eSIM eventually, but they need to get voice-over-LTE laid down first. That probably means 2019.

Apple's new iPhones also aren't quite the only US phones with eSIMs. Google's Pixel 2 and 3 phones have eSIMs, but they can only be used to activate plans on Google's own Fi carrier. Google said in a blog post on Dec. 3 that Truphone, Gigsky, and Sprint will support the Pixel 3's eSIM "in the coming months."

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