Yealink SIP-T48G Administrator's Manual page 617

Step
F8 ACK—User A to Proxy Server
INVITE—User A to Proxy
F9
Server
INVITE—Proxy Server to User
F10
C
180 Ringing—User C to Proxy
F11
Server
180 Ringing—Proxy Server to
F12
User A
200OK—User C to Proxy
F13
Server
200OK—Proxy Server to User
F14
A
F15 ACK— User A to Proxy Server
F16 ACK—Proxy Server to User C
The following figure illustrates successful 3-way calling between Yealink IP phones in
Action
User A sends a SIP ACK to the proxy
server. The ACK message notifies the
proxy server that User A has received
the ACK message.
User A sends a SIP INVITE request to the
proxy server. In the INVITE request, a
unique Call-ID is generated and the
Contact-URI field indicates that User A
requests the call.
The proxy server forwards the SIP
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 sends the ACK
message to User C. The ACK confirms
that the proxy server has received the
200 OK response.
Appendix
Description
603