Which SIP response code indicates a busy user?

The SIP response code that indicates a busy user is 486 Busy Here. This status code specifically communicates that the user agent (the device or application the user is using) is currently busy and cannot accept the new call. It allows the calling party to understand that the user is occupied with another call or is unable to take the call at that moment.

When a caller receives a 486 Busy Here response, it typically means they should either try again later or leave a message, as the intended recipient is not available to engage in a new conversation. This response is crucial in call signaling and management within SIP (Session Initiation Protocol) systems, ensuring proper communication of the user's current availability.

The other response codes, while they signify other important situations, do not indicate that a user is busy. For instance, a 200 OK response indicates that a request was successful, 404 Not Found means the requested resource could not be located, and 180 Ringing signals that the call is being processed but does not imply the user is unavailable.