Question: In an IP Multimedia Subsystem (IMS), what triggers the increment of the 'Number of Communication Forwarding Unconditional (CFU) service procedure' measurement?
Option 1: Transmission by Application Server (AS) of a SIP_INVITE message with URI-C after receiving a SIP_INVITE message with URI-B from CSCF, and execution of URI-B’s CFU logic.
Option 2: Receipt of a SIP_486 (User Busy) message by the AS, prompting a forwarding action.
Option 3: Successful registration of a User Equipment (UE) with the IMS network.
Option 4: Completion of a voice call between two subscribers within the same IMS domain.
Explanation: This is correct because the trigger is specifically tied to the transmission of a SIP_INVITE message with URI-C by the AS after it receives a SIP_INVITE with URI-B from the CSCF and after URI-B’s CFU logic has been executed, directly relating to the unconditional forwarding of communications, which is what CFU is designed for.