Interesting thread - as someone that used to be in carrier space... IMEI - we only really cared about the TAC prefix, as this identifies the device type, which

Interesting thread - as someone that used to be in carrier space... IMEI - we only really cared about the TAC prefix, as this identifies the device type, which is mapped to capabilities for services. IMSI - this is usually in the SIM card (UICC), and mapped out specifically within the uSIM/SIM application inside the card. This is aligned with the Billing/Rate Plan for services that the subscriber is set up with. TMSI - this is usually what the network uses to page you and also deliver singaling over the NAS via the SGs interface for devices that do not support IMS/VoLTE ICCID - this ID's the card itself, for SIM cards, it always starts with 89 as this designates the card as telephony related as a physical UICC - remember, there are other types of UICC's such as CHIP based Credit Cards, which start with a different number. MSISDN - this is the number that you dial and send SMS to - in legacy systems, it can also be referred to as the MDN Fun Fact that was skipped in the article - IMEI's that start with 99 are special, as these indicate that the Device is both GSM/UMTS/LTE and CDMA/EVDO capable, and generally those IMEI's will align closely with the CDMA MEID's, but they were not required to. The "99" range wasn't just Apple, but was used in the early days of dual-mode across most vendors as it helped facilitate session handovers from C2K to any 3GPP based service. For C2K, on the IMSI front, most devices would use IMSI_T (True IMSI based on the SIM card IMSIef) but some used IMSI_M which was based on the legacy MIN. Legacy - there is the ESN in CDMA, but this is very legacy, and was largely superseded by MEID - for Legacy Support, pESN could be derived from MEID, however at the high risk of collisions...