IMSI stands for International Mobile Subscriber Identity. It's a 15-digit number stored on every SIM (physical or eSIM) that tells the network: "This person is one of our subscribers."
Structure
- First 3 digits: Mobile Country Code (UK = 234)
- Next 2-3 digits: Mobile Network Code (EE = 30, O2 = 10, etc.)
- Remaining digits: your unique subscriber ID within that network
Why it matters
When your phone connects to a tower, it sends its IMSI so the network can authenticate you and route traffic. On a multi-network SIM like Fuse, your phone can present different IMSIs depending on which underlying network it's using.
See also
How IMSI matters when picking a UK mobile plan
Understanding IMSI helps you compare mobile providers on a like-for-like basis. Most consumer mobile pricing pages skip the technical detail — knowing the terminology means you can spot when a plan is genuinely better and when it just sounds that way. The Fuse Mobile glossary keeps definitions short and consumer-focused, no jargon-on-jargon.
Fuse Mobile is a UK multi-network data-only eSIM — it gives your phone access to all four UK mobile networks (EE, Three, Vodafone, O2) through a single profile. Where IMSI is relevant to that setup, it's surfaced explicitly: pricing pages show real numbers, the coverage checker shows live per-network signal, and there's no hidden fee structure. See the plans page for the current pricing or read the multi-network eSIM explainer for the technical detail.