Question: In an IP Multimedia Subsystem (IMS), how can a Break-out Gateway Control Function (BGCF) retrieve Number Portability (NP) data for selecting a Media Gateway Control Function (MGCF) for PSTN connection, as per operator policy?
Option 1: Through ENUM/DNS queries using IETF RFC 4769
Option 2: Via direct access to IMS ENUM interfaces, which are updated to support PSTN ENUM service
Option 3: By implementing legacy PSTN/CS-domain protocols (e.g., TCAP) within the BGCF itself
Option 4: Using an interface designated for NP data retrieval at the BGCF, details of which are subject to operator policy and outside this specification
Explanation: The correct answer is Option 4 because the interface used by the BGCF to retrieve NP data is permissible as per operator policy, with its specifics not detailed in the context. This capability exists and is configured based on operator preferences, making Option 4 the most accurate choice.