SIM card, IMSI, IMEI, ICCID and MSISDN background on Mobiles

Your external mobile number is your MSISDN. There are many other numbers.

IMSI = International Mobile Subscriber Identity. This is a unique identifier that defines a subscriber in the wireless world, including the country and mobile network to which the subscriber belongs. It has the format MCC-MNC-MSIN.

MCC = Mobile Country Code (e.g. 310 for USA, 505 for Australia)
MNC = Mobile Network Code (e.g. 410 for AT&T in USA, 02 for Optus in Australia)
MSIN = Sequential serial number.

Click here for these International Mobile Country and Network Codes

The IMSI is one of the pieces of information stored on a SIM card. It can be used in any mobile network that can be connected with other networks. It is typically a 15-digit ID, but it may be shorter in some cases. All signaling and messaging in GSM (2G) and UMTS (3G) and LTE (4G plus 5G) networks uses the IMSI as the primary identifier of a subscriber.
Once registered however, sent as rarely as possible because this protects users from eavesdropping and tracking on the radio interface.

So, instead of using an IMSI, a TMSI Temporary Mobile Subscriber Identity may be used. The TMSI is randomly generated. In LTE, this P-TMSI (Packet-TMSI) is now called the Globally Unique Temporary ID, or the GUTI.

Note, in Australia, where "roaming" between networks does not occur, if your phone is out of range of a cell tower belonging to your network, no calls can be made. Internet access is WiFi only.

ICCID = Integrated Circuit Card ID. This is the identifier of the actual SIM card itself.
i.e. an identifier for the SIM chip. While the information contained on a SIM (e.g. the IMSI) can change, the identity of the SIM itself remains the same.

IMEI is short for International Mobile Equipment Identity and is a unique number given to every single mobile phone, typically found behind the battery.
In Australia, IMEI numbers of cellular phones connected to a GSM network are stored in a database (EIR - Equipment Identity Register) containing all valid mobile phone equipment.
When a phone is reported stolen or is not type approved, the number is marked invalid.
MSISDN is short for Mobile Station International Subscriber Directory Number as appointed by ISDN, the Integrated Services Digital Network.

The MSISDN is the full phone number of a subscriber, including the national country code (e.g. 1 for US, 44 for UK, 61 for Australia etc.). The purpose of the MSISDN is to allow a device to be called.

Some networks allow a SIM card to have multiple MSISDNs e.g. different directory numbers that are local to each country, or perhaps a separate business number and a private number and a fax number.
It can also have multiple IMSI codes.

The MSISDN does not need to be stored on the SIM card. In cases where it is stored on the SIM, the main reason is so that the user can check to see what their MSISDN is (in case they forget). The MSISDN is never signaled to or from the device.

So, as you drive or walk around, Phone Towers receive your IMSI code (International mobile station identity number), and then allocate that device a TMSI (Temporary mobile station identity number) while the phone is within range.

The sequence of operations normally is:
  1. SIM is authenticated in AuC (Authentication Centre) then IMEI is retrieved from device
  2. Device is validated in EIR (Equipment Identity Register) which receives IMEI and optionally IMSI
  3. SIM is registered in HLR/HSS (Home Location Register/Home Subscriber Server) which receives IMSI and optionally IMEI
This is how your Home network knows where you are, and also how each tower knows how to talk to you. Disconnected Numbers policy (Vodafone Australia)

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