A SIM (subscriber identity module) card is used by service providers to identify users on their networks. The number that identifies the SIM card is the International Mobile Subscriber Identity (IMSI) number. This 15-digit number can be written down anywhere on your phone, so you can't find it that easily. You'll have to connect your phone to your computer and run some code to read it. The first three digits will be the mobile country code (MCC), and the next three digits are the mobile network code (MNC). The remaining digits are your unique ID.
Connect your cell phone to your computer's "COM" port with a data cable. Most phones don't come with a data cable, so you'll have to buy this separately. The COM port is a 9-pin port located on your computer tower.
Open up any terminal program you have on your computer. Terminal programs, or terminal emulators, are programs that will give you access to lines of code and will allow you to edit them. You'll have access to codes from programs on your computer or, in this case, codes from devices connected to your computer. If you don't have a terminal program on your computer, you can download one for free online. Hyperterm is a y widely used program for these purposes.
Select the port to which your phone is connected. The number of ports available will depend on how many ports your computer contains.
Type in "AT+CIMI" (no quotes) in the command line.
Press the "Enter" button to see the results. The ISMI number will be displayed in the next line of code. Once you write it down, you can close out of all your programs, and disconnect your phone from the computer.