Question: What triggers the removal of a CSG ID from the Allowed CSG list in a Mobile Equipment (ME)?
Option 1: When the ME powers down and then back up, regardless of IMSI change.
Option 2: When an ATTACH REQUEST message is sent to a cell with that CSG ID and is rejected with cause #25 by the E-UTRAN (E-USS).
Option 3: Automatically after a set period since the last successful TRACKING AREA UPDATE REQUEST.
Option 4: When the USIM's IMSI is updated, even if the ME remains powered on.
Explanation: The correct answer highlights a scenario where the CSG ID is removed from the Allowed CSG list: when an ATTACH REQUEST to a cell with that CSG ID is rejected due to cause #25 by the E-UTRAN. This demonstrates understanding of how network responses influence device behavior in managing allowed Closed Subscriber Group (CSG) IDs for security and access control.