Question: In a private numbering plan, what is the primary identifier used for charging records, authentication, and authorization for services like voice calls, short messages, and PS connections?
Option 1: Underlying MSISDN (Mobile Subscriber Integrated Services Digital Network Number) of each SIM card
Option 2: Corporate phone number without extension
Option 3: Web portal login credentials for special privileges or restrictions
Option 4: Corporate phone number + extension assigned to each user
Explanation: In a private numbering plan, the corporate phone number plus the unique extension assigned to each user serves as the primary identifier for various services, including charging records, authentication, and authorization. This facilitates billing, security, and ease of management without requiring users to be aware of their underlying MSISDNs.