Multi-IMSI SIM: how a multi-network SIM card switches networks

What is an IMSI?

The IMSI (International Mobile Subscriber Identity) is the subscriber identifier stored on the SIM. When a device connects, the network reads this number to identify the SIM and decide whether to let it on. It is up to 15 digits long and has three parts:

• MCC (Mobile Country Code): 3 digits for the country
• MNC (Mobile Network Code): 2 or 3 digits for the network in that country
• the subscriber number (MSIN), which makes up the rest

How a multi-IMSI SIM switches networks

An ordinary SIM carries a single IMSI from its home operator. Abroad, it works through the roaming agreements that operator has signed with local networks. At home, it usually stays on one network, even where a competitor has better signal at the device's location.

On a phone this rarely matters. On a tracker fitted to a truck that crosses borders, or a meter installed in a basement, every area that one network covers poorly leaves a gap in the data.

A multi-IMSI SIM stores several IMSIs from different operators or providers. It is still one card with one ICCID, the card's serial number. Only the identity it presents to the network changes.

An applet, a small program running on the SIM, makes the choice, sometimes following rules pushed over the air (OTA). Based on the country and the networks available, it activates the IMSI that gives good coverage and access close to that of a local subscriber. If the signal is lost or the network rejects the SIM, it can switch to another IMSI, and the device then tries to attach with the new identity.

What switching gives an IoT device

The benefits show most on devices that move, or that sit somewhere nobody visits:

• better coverage, since another network takes over when the first one is weak
• fewer dead zones near borders
• one SIM for a fleet deployed across many countries
• fewer problems in countries that limit permanent roaming

Limits to plan for

Switching IMSI takes time. Between dropping one identity and attaching with the next, anything from a few seconds to a few minutes can pass, and the device is offline for that period. A tracker that reports often will show a short gap in its history.

Switching also depends on the device, which has to re-attach to the network with the new identity. Some trackers only manage this after a restart, others only with a well-configured network search. Test your models before rolling out a whole fleet.

Coverage in each country still depends on the agreements in place with local operators, however many IMSIs the card holds.

Multi-IMSI vs eSIM (SGP.32)

An IoT eSIM (eUICC) managed under the SGP.32 standard downloads complete profiles over the air. A multi-IMSI SIM chooses between identities already stored on the card.

You can combine the two: an eSIM can receive a multi-IMSI profile. That gives you automatic switching day to day, plus the option to change profile without touching the device.

Choosing a multi-network SIM for IoT

Before you sign, ask the provider:

• How many networks can the SIM use in the countries where you deploy?
• Is switching between IMSIs automatic?
• Do your devices reconnect on their own after an IMSI change? Test it with your models.
• Does the console show the country and operator of each SIM?
• What is the price per SIM per month, including platform fees and any commitment?

Atlas M2M multi-IMSI SIMs

Our SIMs are multi-IMSI and switch to the best available operator on their own. The World zone covers 680+ networks in 180+ countries. In the console you can see the country and operator each SIM is connected to.

Plans start at €2 per SIM per month, with no platform fee and no commitment. To try them on your own devices, the free trial gives you 3 SIMs for 30 days with 50 MB in total.