IP Multimedia Services Identity Module

An IP multimedia services identity module (IMSI) is a special serial number that is stored in a device (like your phone) so that your network provider knows which services you are allowed to use. It is like a special card that tells your network which services you can access. Your network provider puts information about all the services you pay for onto the IMSI, so that when you try to use something like a data plan, the network knows that you have paid for it and can let you use it.