USIM vs ISIM
A modern UICC hosts multiple applications. USIM handles cellular authentication (AKA to the HSS/UDM). ISIM handles IMS authentication (IMS-AKA to the S-CSCF). Both live on the same physical or embedded SIM.
| Attribute | USIM | ISIM |
|---|---|---|
| Purpose | Cellular attach / AKA | IMS registration / IMS-AKA |
| Standard | 3GPP TS 31.102 | 3GPP TS 31.103 |
| Keys | K, OP/OPc | IK, CK derived for IMS |
| Identity | IMSI / SUPI | IMPI (private) + IMPU (public) |
| Required for | All cellular | VoLTE, VoNR, IMS services |
Verdict
Devices without ISIM fall back to USIM-derived IMS credentials, which is weaker. Enterprises procuring VoLTE/VoNR handsets should confirm ISIM presence — it is often silently omitted on cheaper devices.