Yealink SIP-T48G Administrator's Manual page 893

Step Action
INVITE—User A to Proxy
F9
Server
INVITE—Proxy Server to User
F10
B
200 OK—User B to Proxy
F11
Server
200 OK—Proxy Server to User
F12
A
F13 ACK—User A to Proxy Server
F14 ACK—Proxy Server to User B
INVITE—User A to Proxy
F15
Server
INVITE—Proxy Server to User
F16
C
180 Ringing—User C to Proxy
F17
Server
180 Ringing—Proxy Server to
F18
User A
Description
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 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
Appendix
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