What is an IMSI?
Every SIM carries an IMSI, a 15 digit number that tells a mobile network which operator the SIM belongs to and which customer it is. The first digits identify the country and the home network. When a SIM from one operator connects to another operator’s network, it is roaming, and the visited network bills the home network for the traffic.
A standard SIM has one IMSI, so it has one home network. A multi-IMSI SIM has two or more, each linked to a different home network.
How a multi-IMSI SIM switches
Switching is handled by a small application on the SIM, or by the provider sending an instruction over the air. The logic is usually one of the following.
- Location based. The SIM notices which country it is in and activates the IMSI that gives local or preferred access there.
- Failure based. If the device cannot register with the active IMSI for a set time, the SIM tries the next one.
- Remote. The provider changes the active IMSI for a whole estate, for example when a better commercial agreement becomes available.
Each switch involves the modem restarting its registration, so a device may be offline for a short time while it happens. Well designed switching rules avoid swapping back and forth.
Why choose multi-IMSI?
- Permanent roaming limits. Some operators restrict or charge more for SIMs that roam on their network all the time. With a local IMSI the device is not seen as a permanent roamer.
- Wider coverage. Different IMSIs can unlock different network agreements, which helps in border areas and in countries where one roaming partner is weak.
- Future proofing. New IMSIs can often be added over the air, so a device installed for ten years is not tied to today’s agreements.
- Lower cost at scale. Traffic can use the most cost effective identity in each region.
Multi-IMSI compared with other options
| Multi network | Multi-IMSI | eSIM (eUICC) | |
|---|---|---|---|
| What is on the SIM | One identity | Several identities, managed by one provider | Full operator profiles that can be downloaded and swapped |
| Who controls changes | The network and the router | The SIM applet and the provider | The device owner, through a profile management platform |
| Standards | Roaming agreements | Provider specific | GSMA SGP.22 and, for IoT, SGP.32 |
| Typical use | UK resilience | International IoT on one contract | Changing supplier or operator without touching the device |
Multi-IMSI and eSIM can also be combined: an eSIM profile can itself be multi-IMSI. See eSIM for M2M connectivity.
Fixed IP on a multi-IMSI SIM
With a basic multi-IMSI product, each IMSI may connect through a different core network, and the IP address can change when the SIM switches. For remote access that defeats the point of a fixed IP. The services we supply route traffic back through a common provider core and APN where the product supports it, so the fixed public or private address stays the same. Always check this before buying a multi-IMSI SIM for a remote access project, and ask us to confirm it for your chosen tariff.
Questions we are often asked
Is a multi-IMSI SIM the same as a multi network SIM?
No. A multi network SIM has one identity with access to several networks. A multi-IMSI SIM has several identities, each with a different home network, and switches between them.
Is multi-IMSI the same as eSIM?
No. Multi-IMSI identities are held by one provider on one SIM. An eSIM can download complete profiles from different operators. The two can be combined.
Will my device notice when the IMSI changes?
The modem re-registers on the network, which usually causes a short loss of connection. Good switching rules keep this rare.
Do I need multi-IMSI for UK only deployments?
Usually not. For UK sites a multi network SIM gives excellent resilience. Multi-IMSI becomes valuable when devices are deployed in several countries or must stay in service for many years.