Specifications

Appendix
425
Step
Action
Description
sends the INVITE request to User C.
F17
180 Ringing—User C to Proxy
Server
User C sends a SIP 180 Ringing response
to the proxy server. The 180 Ringing
response indicates that the user is being
alerted.
F18
180 Ringing—Proxy Server to
User A
The proxy server forwards the 180
Ringing response to User A. User A hears
the ring-back tone indicating that User
C is being alerted.
F19
200OK—User C to Proxy Server
User C sends a SIP 200 OK response to
the proxy server. The 200 OK response
notifies User A that the connection has
been made.
F20
200OK—Proxy Server to User A
The proxy server forwards the SIP 200 OK
response to User A. The 200 OK response
notifies User A that the connection has
been made.
F21
ACK— User A to Proxy Server
User A sends a SIP ACK to the proxy
server. The ACK confirms that User A has
received the 200 OK response. The call
session is now active.
F22
ACK—Proxy Server to User C
The proxy server forwards the ACK
message to User C. The ACK confirms
that the proxy server has received the
200 OK response. The call session is now
active.
F23
REFER—User A to Proxy Server
User A sends a REFER message to the
proxy server. User A performs a transfer
of User B to User C.
F24
202 Accepted—Proxy Server
to User A
The proxy server sends a SIP 202
Accepted response to User A. The 202
Accepted response notifies User A that
the proxy server has received the REFER
message.
F25
REFER—Proxy Server to User B
The proxy server forwards the REFER
message to User B.
F26
202 Accepted—User B to
Proxy Server
User B sends a SIP 202 Accept response
to the proxy server. The 202 Accepted