Question: What action(s) can a system take when a subscriber's pre-set spending limit is reached, according to operator-defined thresholds?
Option 1: Only notify the subscriber via SMS without altering service quality.
Option 2: Automatically upgrade QoS for all IP services to ensure uninterrupted access.
Option 3: Trigger a QoS downgrade and/or restrict access to one, several, or all IP services.
Option 4: Completely suspend all services until the limit is increased by the subscriber.
Explanation: This option aligns with the capability of the system to manage spending limits by adjusting service quality (QoS downgrade) and/or limiting access to specific or all IP services when a pre-set spending threshold is met. This approach balances the need to enforce spending limits while still potentially offering some level of service, thus managing the subscriber's experience and expenditure effectively.