Question: Upon receiving an SGsAP-PAGING-REJECT message with the cause "terminating CS fallback call rejected by the user" before timer Ts5 expires, what action does the VLR take regarding the SGs association state and additional triggers?
Option 1: The VLR changes the SGs association to the SGs-NULL state and marks it with the cause, then triggers User Determined User Busy (UDUB).
Option 2: The VLR sends the paging message on the A/Iu interface and resets timer Ts5.
Option 3: The VLR does not change the SGs association state but triggers User Determined User Busy (UDUB) as specified in call forwarding supplementary services.
Option 4: The VLR stops timer Ts5 and waits for further instructions without taking any additional actions.
Explanation: According to the specifications, if the SGs cause information element indicates "terminating CS fallback call rejected by the user", the SGs association state shall not be changed. However, upon receiving this message, the VLR is required to trigger User Determined User Busy (UDUB) as outlined in the call forwarding supplementary services stage 3 specifications. This precise scenario matches Option 3's description.