eSIM explained: How a phone’s SIM card became software you download

An eSIM is a SIM card turned into software. Instead of a removable plastic card, the phone holds a small chip that can download your carrier’s subscription over the internet. The iPhone Duo, which went on sale in China this week, is eSIM-only, so understanding the technology explains a lot about the launch.

A hotel key card instead of a metal key

Think of a physical SIM as a metal key cut for one door. To change carriers, you swap keys. An eSIM works like a hotel key card: the lock stays the same, and the front desk writes a new code onto it when you check in.

In practice, you buy a plan, scan a QR code or tap a button in an app, and the phone adds the carrier’s subscription. There is no new card to collect and no tiny tray to open.

A chip that can be rewritten, and servers that write to it

The chip is called an eUICC (embedded Universal Integrated Circuit Card). It is built into the phone and can hold several subscriptions. Each one is called a profile and carries the carrier’s details, including the phone number assigned to you.

Profiles arrive through a standard process from the GSMA, the industry body for mobile operators. Three parts matter:

• SM-DP+ (Subscription Manager, Data Preparation): the carrier-side server that prepares your encrypted profile.
• SM-DS (Discovery Server): a “secure notice board”, in the words of an eSIM provider’s explainer on esimdb, that lets the first server reach your device without knowing which network it is on.
• LPA (Local Profile Assistant): software on the phone that downloads the profile and lets you enable, disable or delete it.

In the consumer version, the phone starts the process, which is why you scan a code or press a button yourself. The server then sends the encrypted profile to the chip. Before sensitive data moves, the chip and the servers check each other’s certificates, which trace back to a certificate authority in the GSMA scheme.

From sealed sensors to phones

eSIM began outside phones. A 2019 paper by Maxime Meyer, Elizabeth Quaglia and Ben Smyth reviews the early specifications. In 2010 the standards body ETSI defined a solder-on SIM for machine-to-machine devices such as smart meters and ATMs. In 2015 the GSMA published a remote provisioning architecture for embedded chips, and in February 2016 ETSI introduced the eUICC itself.

The logic was simple: a sensor sealed inside a machine cannot have its SIM swapped by hand. Remote provisioning solved that. According to the paper, the benefits include installing subscriptions remotely, buying the device and subscription separately, and changing operators without replacing hardware. Consumer phones came later, using a version where the user triggers the download rather than a manufacturer pushing it.

Why the Duo’s eSIM-only design matters in China

Apple’s product page for the Duo, as described in our news story, lists it as eSIM-only worldwide. We have not seen Apple explain its reasons.

China is where this matters most, because carriers there had to open the door first. TechNode reported that China’s Ministry of Industry and Information Technology approved commercial eSIM trials for China Mobile, China Unicom and China Telecom on October 13, 2025. The three carriers took different routes: China Unicom took appointments nationwide, China Mobile used app-based registration, and China Telecom required an in-store visit.

Apple’s support page for eSIM in mainland China sets the current rules. It lists the iPhone Duo, iPhone 18 Pro, iPhone 18 Pro Max, iPhone 17e and iPhone Air as models that can use carrier eSIMs there. Activation happens at a carrier store, where staff verify identification, and Apple tells buyers to bring a government ID or passport. Models not sold in mainland China cannot install carrier eSIM profiles there.

So a launch-day buyer does not simply walk out and switch on. Setup has one more step than in the SIM-tray era, and the carriers control that step.

Trade-offs and limits

eSIM is convenient, but it moves some control from you to the carrier and the manufacturer. Several limits are worth knowing:

• Storage is capped. Per Apple, an eSIM-capable iPhone bought in mainland China stores up to two eSIMs. With two already stored, you must delete one to add a travel eSIM.
• Restoring can need a visit. Apple says restoring a deleted mainland eSIM requires a trip to a carrier store.
• Travel has rules. Abroad, you can add a local eSIM if Location Services is on. Non-mainland eSIMs cannot be installed while you are in mainland China.
• Transfers depend on software. With a physical SIM, you move a card between phones. With an eSIM, the move runs through carrier and phone software.
• Security needs scrutiny. Meyer, Quaglia and Smyth stress that remote provisioning must be analyzed carefully, because operators rely on its security properties.

Their paper focused on the machine-to-machine design, so its warnings do not describe a specific flaw in any phone.

What to watch

Three things will show how far this goes. First, whether carriers in China let buyers activate eSIMs outside a store, since TechNode noted that the trial methods differ by carrier. Second, whether more phones follow the Duo and the iPhone Air in dropping the SIM tray. Third, how easily people can move profiles between phones as eSIM-only devices become common.

For the Duo, the price and the screen will get the headlines. The line at the carrier counter is the less visible part of the same story.