Question: How does the network inform the mobile station that a call is progressing in the PLMN/ISDN environment?
Option 1: By sending a progress indicator information element only in an ALERTING message.
Option 2: By sending a progress indicator information element either in an appropriate call control message (e.g., ALERTING or CONNECT) or in the PROGRESS message.
Option 3: By sending a progress indicator information element only in a CONNECT message.
Option 4: By sending a progress indicator information element only in a SETUP message.
Explanation: The network informs the mobile station that a call is progressing in the PLMN/ISDN environment by sending a progress indicator information element either in an appropriate call control message (e.g., ALERTING or CONNECT) if a state change is required, or in the PROGRESS message if no state change is necessary. This allows the mobile station to be aware of the call's progress and react accordingly.