In a SIP REFER-based call transfer, which SIP message is being used by the recipient to notify the originator that the final recipient was successfully contacted? 

Answer options:

A. 200 OK
B. NOTIFY with a message body of 200 OK
C. 202 Accepted
D. 100 Trying
E. 200 BYE

B

The Refer method always begins within the context of an existing call and starts with the originator. The originator sends a Refer request to the recipient (user agent receiving the Refer request) to initiate a triggered Invite request. The triggered Invite request uses the SIP URL contained in the Refer-To header as the destination of the Invite request. The recipient then contacts the resource in the Refer-To header (final-recipient), and returns a SIP 202 (Accepted) response to the originator. The recipient also must notify the originator of the outcome of the Refer transaction--whether the final-recipient was successfully or unsuccessfully contacted. The notification is accomplished using the Notify Method, SIPs event notification mechanism. A Notify message with a message body of SIP 200 . If the call was successful, a call OK indicates a successful transfer, while a body of SIP 503 Service Unavailable indicates an unsuccessful transfer between the recipient and the final-recipient results.