Yealink SIP-T54S Administrator's Manual page 1121

Step Action
F11 200 OK—User B to Proxy Server
F12 200 OK—Proxy Server to User A
F13 ACK—User A to Proxy Server
F14 ACK—Proxy Server to User B
F15 INVITE—User A to Proxy Server
F16 INVITE—Proxy Server to User C
180 Ringing—User C to Proxy
F17
Server
180 Ringing—Proxy Server to
F18
User A
F19 200OK—User C to Proxy Server
Description
INVITE message to User B.
User B sends a SIP 200 OK response to the
proxy server. The 200 OK response
notifies User A that the INVITE is
successfully processed.
The proxy server forwards the 200 OK
response to User A. The 200 OK response
notifies User A that User B is successfully
placed on hold.
User A sends the ACK message to the
proxy server. The ACK confirms that User
A has received the 200 OK response. The
call session is now temporarily inactive.
No RTP packets are being sent.
The proxy server sends the ACK message
to User B. The ACK confirms that the
proxy server has received the 200 OK
response.
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 maps the SIP URI in the
To field to User C. The proxy server sends
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.
Appendix
1095