Question: Under what circumstances, in addition to an information channel being busy, will a Public Land Mobile Network (PLMN) trigger a Network Determined User Busy (NDUB) condition when a call is about to be offered?
Option 1: When the calling subscriber has no active supplementary services.
Option 2: When there is an already ongoing call attempt (incoming or outgoing) that is still in the establishing phase, not yet active.
Option 3: If the called party's UE (User Equipment) is in a location with limited network coverage.
Option 4: When the maximum number of concurrent calls for the 'Call Forwarding' supplementary service has been reached.
Explanation: The NDUB condition is triggered not only by the information channel being busy and reaching the maximum call limit but also by the presence of an ongoing, yet not fully established (i.e., still in the establishing phase), incoming or outgoing call attempt. This ensures that resources are managed efficiently and users are promptly informed of unavailability due to network conditions.