Question: What action does the mobile station take when it receives a progress indicator IE indicating user attachment in a SETUP, CALL PROCEEDING, ALERTING, CONNECT, or PROGRESS message?
Option 1: The mobile station ignores the progress indicator IE and proceeds with call establishment.
Option 2: The mobile station sends an acknowledgement to the network before attaching the user connection for speech.
Option 3: The mobile station attaches the user connection for speech only after receiving a CONNECT message.
Option 4: The mobile station attaches the user connection for speech as soon as an appropriate channel in speech mode is available.
Explanation: According to the specification, when the mobile station receives a progress indicator IE indicating user attachment, it shall attach the user connection for speech as soon as an appropriate channel in speech mode is available. This allows the network to provide in-band tones or announcements before the call reaches the "active" state.