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?

A.
200 OK

B.
NOTIFY with a message body of 200 OK

C.
202 Accepted

D.
100 Trying

E.
200 BYE

Explanation:
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, SIP’s event notification mechanism. A Notify
message with a message body of SIP 200 OK indicates a successful transfer, while a body
of SIP 503 Service Unavailable indicates an unsuccessful transfer. If the call was successful,
a call between the recipient and the final-recipient results.