Question: What triggers the opening of an MMTel Call Detail Record (CDR) in the Charging Data Function (CDF)?
Option 1: Reception of a Charging Data Request [start] message
Option 2: Completion of an MMTel service session
Option 3: Update of subscriber's billing information
Option 4: Expiration of a predefined charging timer
Explanation: The CDF opens an MMTel CDR upon receiving a Charging Data Request [start] message, aligning with the standard approach for handling SIP sessions in telecom charging management, ensuring accurate and timely billing for multimedia telephony services.