Yealink SIP-T48G

Appendix
873
Step
Action
Description
F9
INVITE—User A to Proxy
Server
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.
F10
INVITE—Proxy Server to User
B
The proxy server forwards the mid-call
INVITE message to User B.
F11
200 OK—User B to Proxy
Server
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.
F12
200 OK—Proxy Server to User
A
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.
F13
ACK—User A to Proxy Server
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.
F14
ACK—Proxy Server to User B
The proxy server sends the ACK
message to User B. The ACK confirms
that the proxy server has received the
200 OK response.
F15
INVITE—User A to Proxy
Server
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.
F16
INVITE—Proxy Server to User
C
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.
F17
180 Ringing—User C to Proxy
Server
User C sends a SIP 180 Ringing
response to the proxy server. The 180
Ringing response indicates that the user
is being alerted.
F18
180 Ringing—Proxy Server to
User A
The proxy server forwards the 180
Ringing response to User A. User A
hears the ring-back tone indicating that