From Forum Nokia Wiki

• IMSI stands for International Mobile Subscriber Identity.

• It is a unique 15-digit code used to identify an individual user on a GSM network. It's like a SIM card serial number. Any association with the phone number (MSISDN) that corresponds to it happens on the network side (i.e., operator's servers). ( An exception : MTN South Africa's IMSIs are 14 digits)

• It is associated with all GSM and Universal Mobile Telecommunications System (UMTS) network mobile phone users.

• The IMSI consists of three components:

   A) Mobile Country Code (MCC)
   B) Mobile Network Code (MNC)
   C) Mobile Subscriber Identity Number (MSIN) 

The IMSI is stored in the Subscriber Identity Module i.e what we call SIM or SIM Card.

Internal Related Links

• Usage of CTelephony
• Cell ID with CTelephony
• Reading IMSI in 3rd edition -Synchronously