Question: What triggers the counting of 'Attempted calls, IM CN originated' (CC.AttCallImOrig) in an IMS network?
Option 1: Receipt of a SIP_OK (200) message by the User Equipment (UE)
Option 2: Transmission of a SIP_INVITE message from the MGCF to the UE
Option 3: Establishment of an RTP stream for voice transmission
Option 4: Receipt by the Media Gateway Control Function (MGCF) of an initial SIP_INVITE message
Explanation: The correct trigger for counting 'Attempted calls, IM CN originated' is the receipt by the MGCF of an initial SIP_INVITE message. This signifies the initiation of a call from the IP Multimedia Subsystem (IMS) domain, which is what this measurement is designed to count. The other options occur at different stages of the call setup process or are unrelated to counting attempted calls from the IMS subsystem.