Yealink SIP-T2XP Administrator's Manual page 485

Step Action
C
180 Ringing—User C to Proxy
F17
Server
180 Ringing—Proxy Server to
F18
User A
200OK—User C to Proxy
F19
Server
200OK—Proxy Server to User
F20
A
F21 ACK— User A to Proxy Server
F22 ACK—Proxy Server to User C
Description
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.
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
469