Yealink SIP-T2 SERIES Administrator's Manual page 1075

Step Action
F2 INVITE—Proxy Server to User B
180 Ringing—User B to Proxy
F3
Server
180 Ringing—Proxy Server to
F4
User A
302 Move Temporarily—User B to
F5
Proxy Server
F6 ACK—Proxy Server to User B
302 Move Temporarily—Proxy
F7
Server to User A
F8 ACK—User A to Proxy Server
F9 INVITE—User A to Proxy Server
F10 INVITE—Proxy Server to User C
180 Ringing—User C to Proxy
F11
Server
180 Ringing—Proxy Server to
F12
User A
Description
The port on which User B is prepared

to receive the RTP data is specified.
The proxy server maps the SIP URI in the To
field to User B. The proxy server sends the
INVITE message to User B.
User B 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 B is
being alerted.
User B sends a SIP 302 Moved Temporarily
message to the proxy server. The message
indicates that User B is not available at SIP
phone B. User B rewrites the contact-URI.
The proxy server sends a SIP ACK to User B,
the ACK message notifies User B that the
proxy server has received the 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.
Appendix
1051