Yealink SIP-T54S Administrator's Manual page 1108

Administrator's Guide for SIP-T2 Series/T19(P) E2/T4 Series/T5 Series/CP860/CP920 IP Phones
Step
F9 INVITE—User A to Proxy Server
F10 INVITE—Proxy Server to User B
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
1082
Action
User A sends a mid-call INVITE request to
the proxy server with new SDP session
parameters, which are used to place the
call on hold.
The proxy server forwards the mid-call
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 was
successfully processed.
The proxy server forwards the 200 OK
response to User A. The 200 OK response
notifies User B is successfully placed on
hold.
User A sends an 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 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
Description