TMSIs
Temporary Mobile Subscriber Identity (TMSI) is a temporary identifier assigned by a mobile network to a subscriber to protect IMSI on the radio interface. It is used in GSM and its successors to identify the subscriber during signaling such as attach, routing area updates, and paging, without transmitting the permanent IMSI.
In GSM/UMTS networks, the TMSI is allocated by the Visiting Location Register (VLR) or equivalent network element
TMSIs may have additional variants in specialized domains: P-TMSI (Packet TMSI) is used in GPRS/GSM data services
Security and privacy concerns include the fact that while TMSI reduces IMSI exposure, it does not provide