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.