Glossary definition of IMSI

Also known as: International Mobile Subscriber Identity

IMSI (International Mobile Subscriber Identity) is a unique identifying number stored in the phone's SIM. The IMSI relates directly to one mobile phone number, and it is an internal subscriber identity used only by the network to allow phone networks to track the location of phones and to assign call charges.

IMSI codes for subscriber numbers belonging to a mobile phone network are kept in its home location register, however, the IMSIs of roaming phones belonging to other networks are stored in its VLR. A SIM only transmits its IMSI after the correct PIN has been entered.