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.