# ICCID, IMSI, MSISDN and eUICC: SIM numbers explained

> What the ICCID, IMSI and MSISDN numbers on a SIM mean, which one you need and when, plus eUICC, eSIM, ISIM and multi-IMSI SIMs in plain terms.

Fleets & developers · 27 October 2026

A SIM carries several numbers, and IoT portals, device settings and support teams each ask for a different one. This is what each means and when you need it.

## The short version

| Number | What it identifies | Where you see it | When you need it |
|---|---|---|---|
| ICCID | The SIM card itself | Printed on the card; in the portal; in the device's status page | Activating, pausing or ceasing a SIM; matching a SIM to a device |
| IMSI | The subscription on the mobile network | Device diagnostics; network logs | Troubleshooting registration with an engineer |
| MSISDN | The SIM's phone number | The portal | Sending SMS commands to a device |

## ICCID: the SIM's serial number

The **ICCID** (integrated circuit card identifier) is the SIM's own serial number: usually 19 or 20 digits starting 89, printed on the card and readable by the device. It identifies the physical SIM or the eSIM profile, whichever network it is using. In an IoT fleet it is the number you track: it is how a SIM is found in a portal, labelled against the device it sits in, and activated or paused. Most routers and trackers show it on their status or SIM page.

## IMSI: the subscription on the network

The **IMSI** (international mobile subscriber identity) identifies the subscription to the mobile network when the SIM attaches. It starts with the country and network codes of the operator that issued it. You rarely need it day to day, but an engineer may ask for it when a device is not registering. A **multi-IMSI SIM** holds more than one IMSI and can switch between them, which is one of the ways IoT SIMs reach several networks.

## MSISDN: the SIM's phone number

The **MSISDN** is the phone number attached to the SIM, in international format. A data-only IoT SIM still has one, because SMS uses it: if a tracker or alarm communicator takes configuration commands by text message, the MSISDN is the number you send them to. Every ThingSIM supports SMS.

## eSIM, eUICC and ISIM

- **eSIM** is the everyday name for a SIM profile downloaded to the device instead of arriving on a card.
- **eUICC** is the chip that makes that possible: an embedded universal integrated circuit card that can hold one or more downloadable profiles. Strictly, the eUICC is the hardware and the eSIM is the profile on it, which is why "eUICC vs eSIM" is often a question of wording rather than technology.
- **ISIM** is a SIM function built into the device's main processor or modem rather than a separate chip, the next step after eSIM.

How profiles reach an eUICC is set by GSMA specifications; see [IoT eSIM standards explained](/iot-esim-standards-sgp02-sgp22-sgp32).

## ThingSIM

The portal lists every SIM by ICCID, with its usage and status, and the same identifiers are available through the [API](/developers). Every ThingSIM is available as a physical trio card or as an eSIM delivered by QR code.

## Common questions

### What is an ICCID number?

The SIM's serial number: usually 19 or 20 digits starting 89, printed on the card and readable by the device. It identifies the SIM itself, and is the number used to find, activate or pause it.

### Is the ICCID the same as the IMSI?

No. The ICCID identifies the SIM card or eSIM profile; the IMSI identifies the subscription to the mobile network that the SIM presents when it attaches.

### What is an MSISDN?

The phone number of a SIM, in international format. On a data-only IoT SIM it is used for SMS, for example to send configuration commands to a tracker.

### What is the difference between eUICC and eSIM?

The eUICC is the embedded chip that can hold downloadable SIM profiles; an eSIM is the profile itself. In everyday use the two terms are often used interchangeably.

## Recommended plan

50 MB, UK 4-network: £0.90 a month per active SIM, ex VAT.

Source: https://thingsim.com/iccid-imsi-msisdn-euicc