The IMSI (International Mobile Subscriber Identity) is a unique number of up to 15 digits that identifies a mobile subscription to cellular networks worldwide. In the world of SIM & eSIM, it is stored in the subscriber profile on a physical SIM card or a downloaded eSIM profile, and the network uses it to find the subscriber record, authenticate the device and decide which services and roaming rules apply.
The IMSI identifies the subscription, not the phone and not the phone number. The handset has its own identifier, the IMEI, and the number other people dial is the MSISDN. Because the IMSI is a permanent and globally unique identity, networks send it over the air as rarely as possible and replace it with temporary identities to protect privacy.
Structure of the IMSI number
An IMSI consists of three fields, defined in the ITU-T E.212 numbering plan and the 3GPP specifications. The first two fields identify the operator that issued the subscription, and the last field identifies the individual subscriber within that operator.
| Field | Length | Meaning | Example value |
|---|---|---|---|
| MCC (mobile country code) | 3 digits | Country or region of the home network | 310 for the United States |
| MNC (mobile network code) | 2 or 3 digits | Home operator within the country | A two-digit or three-digit code |
| MSIN (subscriber number) | Up to 10 digits | Individual subscription at the operator | Assigned by the operator |
The combination of MCC and MNC is called the home network identity, often shown as the PLMN code. Together the fields never exceed 15 digits. Whether the MNC has two or three digits is not visible from the number alone; devices learn it from a separate field on the SIM, which matters when software splits an IMSI into its parts.
How the network uses the IMSI
The IMSI is the key to the subscriber database of the home operator, called the HLR in older networks, the HSS in LTE and the UDM in 5G. When a device attaches for the first time, the visited network reads the MCC and MNC to locate the home network and requests authentication vectors for that IMSI.
Authentication relies on a secret key, Ki, stored together with the IMSI in the SIM and in the operator’s authentication center. The network sends a random challenge, the SIM computes a response with the key, and both sides derive session encryption keys. The secret key never leaves the SIM, which is why copying an IMSI alone does not allow someone to use the subscription.
After a successful attach, the network assigns a temporary identity and uses it for paging and later signaling. The following table shows how identities have evolved across generations.
| Generation | Permanent identity | Temporary identity | Privacy of the permanent identity |
|---|---|---|---|
| GSM and UMTS | IMSI | TMSI or P-TMSI | Sent in clear on first attach |
| LTE | IMSI | GUTI | Still sent in clear when requested |
| 5G standalone | SUPI, usually IMSI-based | 5G-GUTI | Encrypted as SUCI before sending |
IMSI compared with other mobile identifiers
Several numbers are associated with every mobile connection, and they are easy to confuse. The IMSI belongs to the subscription profile, the ICCID to the SIM itself, the IMEI to the device and the MSISDN to the dialable phone number.
| Identifier | What it identifies | Where it is stored | Visible to the user |
|---|---|---|---|
| IMSI | Subscription at the home operator | SIM or eSIM profile and operator database | Rarely, through diagnostic tools |
| ICCID | The physical SIM or eSIM profile | SIM, printed on the card | Yes, in settings or on the card |
| IMEI | The mobile device hardware | Device firmware | Yes, through a dial code or settings |
| MSISDN | The phone number | Operator database, optionally the SIM | Yes |
| SUCI | Encrypted form of the SUPI | Generated per request in 5G | No |
The relationship between these numbers is flexible. An operator can change the phone number without changing the IMSI, and a SIM swap moves the phone number to a new card that usually carries a new IMSI and ICCID. Some multi-IMSI SIM cards even hold several IMSIs and choose one depending on the country, which roaming providers use to obtain better local tariffs.
IMSI in SIM cards, eSIM and iSIM
On a classic SIM card, the IMSI is written during personalization at the factory, together with the secret key. With eSIM, the IMSI and key are part of an eSIM profile that a provisioning server, the SM-DP+, prepares and downloads securely into the eUICC chip. An eUICC can hold several profiles and therefore several IMSIs, but typically only one profile is active at a time per radio.
The same logic applies to iSIM, where the secure element is integrated into the device processor. For the network, there is no difference in how the IMSI is used; only the method of delivering it to the device changes.
Privacy risks and IMSI catchers
Because the IMSI is permanent, anyone who can capture it can track a subscriber over time. An IMSI catcher is a fake base station that imitates a legitimate network, persuades nearby phones to connect and then requests their permanent identity. Older generations allowed this request without strong protection, and some devices can be forced to fall back to GSM, where mutual authentication is missing.
- 5G concealment: in standalone 5G, the device encrypts the subscriber identity with the home operator’s public key, so only the SUCI travels over the air.
- Temporary identities: frequent reallocation of GUTI values makes passive tracking harder.
- Downgrade risk: protections help only if the device does not accept older, weaker network types.
- Data handling: IMSI values in logs, apps and support systems are personal data and should be protected accordingly.
A common misunderstanding is that knowing an IMSI allows calls to be intercepted or a SIM to be cloned. Interception and cloning require the secret key or weaknesses in older encryption, not the identity number alone; the IMSI mainly enables identification and location tracking.
A non-technical view
Think of the IMSI as the account number of your mobile plan. Friends call you on your phone number. Behind the scenes, though, the network finds your account by this hidden number on your SIM.
Abroad, a foreign network reads the first digits. They show which country and which operator you belong to. The network then asks your home operator whether you may connect. Because the number is permanent, modern networks hide it. They use short-lived nicknames instead.
A concrete example
A traveler from Germany lands in Spain and switches on a phone with a German SIM. The Spanish network reads the IMSI, sees MCC 262 for Germany and the home operator’s MNC, and contacts the German operator’s subscriber database. The home network checks that roaming is allowed, provides authentication data, and the phone is registered within seconds.
Later, a company fleet manager deploys 300 tracking devices with multi-IMSI SIM cards. When a truck crosses into another country, the SIM switches to an IMSI from a partner operator in that region, and the fleet avoids expensive roaming charges while every device stays reachable through the same management platform.