In a Session Initiation Protocol (SIP) call, a pre-answer is a provisional response sent by the callee (the person being called) to the caller to indicate that the call is being processed and that a final response will be sent shortly.
When a SIP phone receives an incoming call, it will send a provisional response, such as a "ringing" tone, to the caller to let them know that the call is being processed. This provisional response is also known as a pre-answer.
The pre-answer is important because it lets the caller know that the call has been received and is being processed, even if the final response, such as an answer or decline, has not been sent yet. It can also be used to convey additional information, such as the reason for the delay in answering the call.
In summary, a pre-answer is a provisional response sent by the callee in a SIP call to indicate that the call is being processed and to provide some information about the status of the call to the caller.