From Forum Nokia Wiki

• The TIMSI stands for Temporary International Mobile Subscriber Identity, which is a pseudo-random number generated from the IMSI number.

• The TIMSI is utilized in order to remove the need to transmit the IMSI over-the-air. This helps to keep the IMSI more secure.

• To track a GSM user via IMSI or TIMSI, an eavesdropper must intercept the GSM network communication where the TIMSI is initially negotiated.

• In addition, because the TIMSI is periodically renegotiated, the eavesdropper must intercept each additional TIMSI re-negotiation session.