iSIM (Integrated SIM)

The IP Multimedia Services Identity Module is the application that runs on a UICC alongside the USIM and stores the IMS subscriber identity (IMPI/IMPU), home network domain, and IMS authentication credentials. The iSIM enables SIM-based authentication to IMS services such as VoLTE/VoNR/RCS, complementing SIM-based authentication to the underlying mobile network.

Categories: Radio Access NetworkRoaming and InterconnectIdentity and Subscriber

iSIM in context

The radio access network is the layer where mobile devices attach to the operator's infrastructure. Attacks in this layer include IMSI catching, rogue base stations, downgrade attacks and jamming, and defenses hinge on mutual authentication, integrity-protected signaling and Open RAN supply-chain hygiene.

Roaming and interconnect are where two operators, often with an IPX or GRX carrier in between, exchange signaling and user-plane traffic. Trust boundaries here are the primary attack surface for location tracking, SMS interception and fraud, which is why GSMA now mandates SEPP with PRINS on 5G interconnect.

To place iSIM in the wider telecom-security picture, review SUPI, AKA, IMSI Catcher (Cell Site Simulator), SUPI Concealment, Authentication and AUSF.

Related terms

Equipment

SIM, eSIM and provisioning (UICC, eUICC, SM-DP+, OTA)

Articles in the P1 mobile network security guide

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