Question: What is a required capability for E.164 numbers within the IP Multimedia (IM) Core Network (CN) subsystem, as per telecom service principles?
Option 1: Support for Multiple Device Sharing
Option 2: Enhanced Security Authentication Protocols
Option 3: Number Portability across Different Networks
Option 4: Mandatory Call Forwarding to Voicemail
Explanation: Number portability is crucial for user convenience, allowing subscribers to retain their phone numbers (E.164 numbers) when switching between different network providers. This capability ensures continuity of service and is a key aspect of telecom regulations, aligning with the principles of service provision in the IM CN subsystem.