Guide
What is an eSIM?
A SIM is the small chip that tells a mobile network who you are. An eSIM is the same idea, built into the device and set up by download instead of with a card. It started in watches and phones; now it is making its way into things too small for a card slot at all.
In short. An eSIM is a SIM chip fixed inside the device; you add your operator by downloading a profile, often by scanning a code, instead of inserting a card. Apple calls it an industry-standard digital SIM, and an iPhone can manage up to eight or more of them. A cellular Apple Watch shares the iPhone’s number through the same carrier. Small devices like it because a card slot takes space and is one more opening for water.
How it works
The chip is fixed inside the device. Instead of inserting a card, you download a profile from a mobile operator, often by scanning a code or through an app. One eSIM can hold more than one profile, and the device can switch between them. Apple describes it as an industry-standard digital SIM built into the phone, which does away with the physical card.
In a phone
On an iPhone, eSIM works from the iPhone XS and XR on, with an operator or a travel provider that supports it, and Apple says a phone can manage up to eight or more eSIMs, so a work line, a personal line and a travel plan can live side by side. Some regions differ: Apple notes that only certain models support eSIM in mainland China, for example.
In a watch
A cellular Apple Watch shares your iPhone’s number, and Apple’s conditions are telling: the watch and the phone must use the same carrier, except watches set up for children; the plan must come from a supported carrier; prepaid and some older accounts are not supported. Cellular watches are also sold in regional versions, and Apple says no single model covers all cellular bands worldwide. A watch with its own connection is still, in this sense, an extension of your phone line.
Why small devices use it
- Space. No card slot, which matters in a watch and even more in a ring.
- Sealing. No slot means one opening fewer for water and dust: what water ratings mean.
- Set-up in software. The operator can be chosen and changed without a card.
In a ring
Lychnos Ring works differently from a watch: its eSIM carries data only, has no phone number, and uses LTE-M, a mobile network made for small devices. That is what lets it reach its assistant without your phone, and why it does not depend on your own mobile operator. More in what LTE-M is, and how a ring compares with a watch in smart ring or smartwatch?
Not a payment chip
An eSIM and a contactless card both replaced a plastic card with a chip, but they do different jobs. The eSIM identifies the device to a mobile network. Paying at a terminal uses a separate payment chip, an NFC antenna and a token your bank issues for that device, and it works even with no mobile signal. A ring can carry both, one for its connection and one for the till: how tap to pay works and where payments in Europe are heading.
When you travel
With an eSIM you can add a local or travel profile without a card, where your device and your operator allow it. A ring’s eSIM is set up for you: where it works abroad depends on the networks it can use, and for Lychnos Ring that list comes with the final offer. Why it matters: a phone-free assistant.
Sources, read 4 November 2026: Apple Support, set up eSIM on iPhone (published 22 October 2026) and set up cellular on Apple Watch (published 14 September 2026).
Questions
How many eSIMs can an iPhone hold?
Apple says an iPhone can manage up to eight or more eSIMs. eSIM works from the iPhone XS and XR on, with an operator or travel provider that supports it.
Does an Apple Watch with cellular have its own number?
It shares your iPhone’s number: Apple says the two share the same number and must use the same carrier, except watches set up for children. Prepaid and some older accounts are not supported.
Can an eSIM pay at a terminal?
No. An eSIM tells a mobile network who you are. Paying at a terminal uses a separate payment chip with an NFC antenna and a payment token from your bank. A ring can carry both: Lychnos Ring is planned with an eSIM for its connection and a payment chip for tap to pay.
Does an eSIM work in every country?
It works wherever the operator behind its profile has coverage or roaming, and where the device supports the local bands. For Lychnos Ring, the list of countries comes with the final offer.