Yealink SIP-T2XP Administrator's Manual page 477

Step Action
302 Move Temporarily—Proxy
F7
Server to User A
F8 ACK—User A to Proxy Server
INVITE—User A to Proxy
F9
Server
INVITE—Proxy Server to User
F10
C
180 Ringing—User C to Proxy
F11
Server
180 Ringing—Proxy Server to
F12
User A
200OK—User C to Proxy
F13
Server
200OK—Proxy Server to User
F14
A
F15 ACK— User A to Proxy Server
F16 ACK—Proxy Server to User C
Description
ACK message.
The proxy server forwards the 302
Moved Temporarily message to User A.
User A sends a SIP ACK to the proxy
server. The ACK message notifies the
proxy server that User A has received
the ACK message.
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 forwards 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.
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.
Appendix
461