An international mobile subscriber identity, or IMSI, is a unique ID number associated with a cellphone user. It's stored in the phone's SIM card and used to identify the user and plan to the cellphone network. Some phones have built-in options to display the IMSI number and other identification numbers associated with the phone, and some smart phone IMSI check apps can display the IMSI on demand.
Understanding the IMSI Number
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An IMSI number uniquely identifies a cellphone subscriber and plan, similar to how a routing number and bank account number uniquely identify a bank customer and account. It's usually 15 digits long.
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It includes a five-digit or six-digit prefix identifying the cellphone carrier. The rest of the number then uniquely identifies the account within that carrier's systems. The number is stored on the subscriber identity module, or SIM, which is usually a removable smart card inserted into a cellphone.
If you transfer a SIM card from one compatible cellphone to another, the IMSI will naturally transfer with the SIM card. If a cellphone has multiple SIM cards, such as to connect to multiple networks, each will have its own IMSI.
When you join a cellphone network, whether it's your home network or a network on which your phone is roaming, your phone broadcasts the IMSI from your SIM card to identify you to the network.
Other Phone Identifiers
The IMSI is not your phone number, which is sometimes referred to in cellphone company lingo as the MSISDN, or mobile subscriber integrated services digital network number. You can't dial someone's IMSI to make a call, and you generally don't need to know your own IMSI or give it to anyone.
It's also distinct from the international mobile equipment identity number, or IMEI, which identifies your phone itself rather than your SIM or your account.
IMSI Lookup on Your Phone
Depending on your phone, you may be able to enter a special sequence of keystrokes to access your IMSI number. This may work by using an internal command within the phone or by contacting your cellphone carrier's servers to request the number. The codes involved vary from phone to phone and carrier to carrier, so check with your phone's manufacturer or your carrier, or search online to find out how to access the IMSI.
On some Android phones, you can also access your IMSI through the Settings menu. Tap the "About" entry in the menu, then tap "Status" or "Phone Identity." The IMSI will be listed under IMSI if it's available through this menu.
You can also get the IMSI on an Android phone by installing a specialized app. Make sure to install one you trust, since it will have access to data about your phone.
Check With Your Carrier
Your IMSI may also be included in information you've received from your carrier when you activated your account or purchased your phone or SIM card. Check the information you received from the carrier when you created your account and activated your phone and card. It may also be available online through the carrier's website or smart phone app.
If you don't see the IMSI, try contacting your carrier to inquire what the code is. Note that you generally don't need to know the IMSI to use your phone.