Yealink SIP-T48G Administrator's Manual page 880

Administrator's Guide for SIP-T2 Series/T19(P) E2/T4 Series/CP860 IP Phones
Step
C
180 Ringing—User C to Proxy
F17
Server
180 Ringing—Proxy Server to
F18
User A
200OK—User C to Proxy
F19
Server
200OK—Proxy Server to User
F20
A
F21 ACK— User A to Proxy Server
F22 ACK—Proxy Server to User C
REFER—User A to Proxy
F23
Server
202 Accepted—Proxy Server
F24
to User A
F25 REFER—Proxy Server to User B
202 Accepted—User B to
F26
Proxy Server
860
Action
sends the INVITE request to User C.
User C sends a SIP 180 Ringing
response to the proxy server. The 180
Ringing response indicates that the user
is being alerted.
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.
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.
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.
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.
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.
User A sends a REFER message to the
proxy server. User A performs a transfer
of User B to User C.
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.
The proxy server forwards the REFER
message to User B.
User B sends a SIP 202 Accept response
to the proxy server. The 202 Accepted
Description