A SIM card has 3 identifying characteristics. Please take note of these as they are important in many circumstances, and will also be required when logging a SIM Support request with us.
SIM MSISDN (Mobile Station International Subscriber Directory Number) - The SIM MSISDN, also known as the GSM, CTN or telephone number, consists of 15 digits and is primarily used to identify a mobile phone number internationally. An MSISDN is defined by the E.164 numbering plan. This number includes a country code and a National Destination Code which identifies the subscriber's operator.
SIM IMSI (International Mobile Subscriber Identity) - The SIM IMSI is a unique number, usually 15 digits, which identifies a GSM/MSISDN number. The IMSI is stored in the SIM in the mobile/device which in turn send it to the appropriate network to be verified.
SIM ICCID (Integrated Circuit Card Identifier) - The SIM ICCID number is up to 22 digits long and serves as the SIMs international identifier. It is most often printed on the body of the SIM.
you can see some sample MSISDN, IMSI and ICCID numbers, along with their formatting here.