Question: In a mobile terminated call with non-exhaustive compatibility information (e.g., call originated from a PSTN), how is subscription checking handled in a Single Numbering Scheme?
Option 1: Implicit subscription check is performed by the HLR using called number information.
Option 2: Subscription check is skipped, and calls are automatically allowed.
Option 3: The VLR/MSC performs the subscription check after the UE responds to the call set-up, or passes calls 'unfiltered' at the network operator's discretion.
Option 4: The call is immediately released due to insufficient information.
Explanation: In a Single Numbering Scheme with non-exhaustive compatibility information (e.g., from a PSTN), the Basic Service cannot be deduced until the UE responds. Thus, the HLR cannot perform an implicit subscription check. Instead, the VLR/MSC either performs the subscription check after the UE's response or allows calls to pass through without filtering, based on the network operator's policy, balancing security and service accessibility under ambiguous call conditions.