Question: What action does the network take when it cannot support a change to the requested call mode during an in-call modification?
Option 1: The network sends a MODIFY ACCEPT message with the new bearer capability.
Option 2: The network releases resources reserved for the alternation, sends a MODIFY REJECT message with the old bearer capability and cause #58 "bearer capability not presently available", and enters the "active" state.
Option 3: The network initiates call clearing procedures with cause #102 "recovery on timer expiry".
Option 4: The network stops timer T323 and releases resources reserved for the alternation.
Explanation: According to the specification, when the network cannot support a change to the requested call mode during an in-call modification, it must release resources reserved for the alternation, send a MODIFY REJECT message with the old bearer capability and cause #58 "bearer capability not presently available", and enter the "active" state. This ensures that the call remains in a stable state and resources are properly released.