Action Triggering the 180 Ringing Response

C.

In the context of Session Initiation Protocol (SIP), the 180 Ringing response is used by the User Agent Server (UAS) to indicate that it is attempting to alert the called party about an incoming call. The 180 Ringing response is typically generated by the UAS in response to a SIP INVITE message sent by the User Agent Client (UAC).

Out of the options given, the action that triggers the 180 Ringing response in a UAC is option C: the generation of a local ring-back tone. When the UAC sends a SIP INVITE message to the UAS, the UAS starts the ringing timer and sends a 180 Ringing response back to the UAC. The UAS then starts generating a local ring-back tone to indicate to the calling party that the call is being processed.

The UAC receives the 180 Ringing response and starts the ringing timer. It also starts generating a local ring-back tone to indicate to the calling party that the call is in progress. The UAC continues to play the ring-back tone until it receives a final response from the UAS, such as a 200 OK response indicating that the call has been established, or a 4xx or 5xx response indicating that the call cannot be completed as dialed.

Therefore, the correct answer is C: the generation of a local ring-back tone triggers the 180 Ringing response in a UAC.