Question: What is a mandatory co-requisite for accessing the rest of the 3GPP system when utilizing an ISIM (IMS Subscriber Identity Module) for IMS services over GERAN/UTRAN/EUTRAN or I-WLAN?
Option 1: A second ISIM with enhanced security features
Option 2: A USIM (Universal Subscriber Identity Module) to be present alongside the ISIM
Option 3: An additional application on the UICC for general telecommunications services
Option 4: A specific IMS-only UICC separate from the one holding the ISIM
Explanation: When an ISIM is used to access IMS over certain networks, a USIM also needs to be present to facilitate access to the rest of the 3GPP system, highlighting the complementary role of USIM in providing comprehensive network access.