IMSI (International Mobile Subscriber Identity)
Updated
The IMSI is the identity of a mobile subscription, stored on the SIM and sent to the network when the device registers. It has at most 15 digits: a 3-digit mobile country code (MCC), a 2- or 3-digit mobile network code (MNC) and a subscriber number (MSIN). It is defined by ITU-T E.212 and 3GPP TS 23.003.
In practice
The MCC and MNC of the IMSI tell visited networks which home network the SIM belongs to, which is why some countries can detect and restrict SIMs that stay in permanent roaming.