Question: What is the primary purpose of a Mobile Station Roaming Number (MSRN) in a PSTN/ISDN routing context?
Option 1: To uniquely identify an MS for subscriber dialling.
Option 2: To route calls directed to an MS, temporarily allocated by the VLR and passed to the Gateway MSC.
Option 3: To provide a permanent identifier for an MS within a visited PLMN.
Option 4: To facilitate international roaming between different numbering areas.
Explanation: The primary purpose of an MSRN is to route calls directed to an MS. It is temporarily allocated by the VLR and passed to the Gateway MSC, which uses it to direct calls to the correct Visited MSC. This allows for efficient routing of calls within a visited PLMN, even when the MS is roaming.